pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

/// This file declares a "shadow hierarchy" of concrete classes which extend
/// the kernel class hierarchy, adding methods and fields needed by the
/// BodyBuilder.
///
/// Instances of these classes may be created using the factory methods in
/// `ast_factory.dart`.
///
/// Note that these classes represent the Dart language prior to desugaring.
/// When a single Dart construct desugars to a tree containing multiple kernel
/// AST nodes, the shadow class extends the kernel object at the top of the
/// desugared tree.
///
/// This means that in some cases multiple shadow classes may extend the same
/// kernel class, because multiple constructs in Dart may desugar to a tree
/// with the same kind of root node.

import 'dart:core' hide MapEntry;

import 'package:kernel/ast.dart' as kernel show Expression, Initializer;

import 'package:kernel/ast.dart' hide InvalidExpression, InvalidInitializer;

import 'package:kernel/clone.dart' show CloneVisitor;

import 'package:kernel/type_algebra.dart' show Substitution;

import '../../base/instrumentation.dart'
    show
        Instrumentation,
        InstrumentationValueForMember,
        InstrumentationValueForType,
        InstrumentationValueForTypeArgs;

import '../fasta_codes.dart'
    show
        messageSwitchExpressionNotAssignableCause,
        messageVoidExpression,
        noLength,
        templateCantInferTypeDueToCircularity,
        templateCantUseSuperBoundedTypeForInstanceCreation,
        templateForInLoopElementTypeNotAssignable,
        templateForInLoopTypeNotIterable,
        templateSwitchExpressionNotAssignable;

import '../problems.dart' show unhandled, unsupported;

import '../source/source_class_builder.dart' show SourceClassBuilder;

import '../source/source_library_builder.dart' show SourceLibraryBuilder;

import '../type_inference/inference_helper.dart' show InferenceHelper;

import '../type_inference/interface_resolver.dart' show InterfaceResolver;

import '../type_inference/type_inference_engine.dart'
    show
        FieldInitializerInferenceNode,
        IncludesTypeParametersCovariantly,
        InferenceNode,
        TypeInferenceEngine;

import '../type_inference/type_inferrer.dart'
    show
        ExpressionInferenceResult,
        TypeInferrer,
        TypeInferrerDisabled,
        TypeInferrerImpl;

import '../type_inference/type_inference_listener.dart'
    show
        AsExpressionTokens,
        AssertInitializerTokens,
        AssertStatementTokens,
        AwaitExpressionTokens,
        BlockTokens,
        BoolLiteralTokens,
        BreakStatementTokens,
        ContinueStatementTokens,
        ConditionalExpressionTokens,
        ContinueSwitchStatementTokens,
        DoStatementTokens,
        DoubleLiteralTokens,
        EmptyStatementTokens,
        ExpressionStatementTokens,
        ForInStatementTokens,
        ForStatementTokens,
        IfNullTokens,
        IfStatementTokens,
        IntLiteralTokens,
        IsExpressionTokens,
        IsNotExpressionTokens,
        ListLiteralTokens,
        LogicalExpressionTokens,
        MapLiteralTokens,
        NotTokens,
        NullLiteralTokens,
        RethrowTokens,
        ReturnStatementTokens,
        StringLiteralTokens,
        SuperInitializerTokens,
        SwitchCaseTokens,
        SwitchStatementTokens,
        ThisExpressionTokens,
        ThrowTokens,
        CatchStatementTokens,
        TryFinallyTokens,
        WhileStatementTokens,
        YieldStatementTokens,
        NamedExpressionTokens,
        TypeInferenceListener;

import '../type_inference/type_promotion.dart'
    show TypePromoter, TypePromoterImpl, TypePromotionFact, TypePromotionScope;

import '../type_inference/type_schema.dart' show UnknownType;

import '../type_inference/type_schema_elimination.dart' show greatestClosure;

import '../type_inference/type_schema_environment.dart'
    show TypeSchemaEnvironment, getPositionalParameterType;

import 'body_builder.dart' show combineStatements;

import 'kernel_expression_generator.dart' show makeLet;

/// Indicates whether type inference involving conditional expressions should
/// always use least upper bound.
///
/// A value of `true` matches the behavior of analyzer.  A value of `false`
/// matches the informal specification in
/// https://github.com/dart-lang/sdk/pull/29371.
///
/// TODO(paulberry): once compatibility with analyzer is no longer needed,
/// change this to `false`.
const bool _forceLub = true;

/// Computes the return type of a (possibly factory) constructor.
InterfaceType computeConstructorReturnType(Member constructor) {
  if (constructor is Constructor) {
    return constructor.enclosingClass.thisType;
  } else {
    return constructor.function.returnType;
  }
}

List<DartType> getExplicitTypeArguments(Arguments arguments) {
  if (arguments is ArgumentsJudgment) {
    return arguments._hasExplicitTypeArguments ? arguments.types : null;
  } else {
    // This code path should only be taken in situations where there are no
    // type arguments at all, e.g. calling a user-definable operator.
    assert(arguments.types.isEmpty);
    return null;
  }
}

/// Information associated with a class during type inference.
class ClassInferenceInfo {
  /// The builder associated with this class.
  final SourceClassBuilder builder;

  /// The visitor for determining if a given type makes covariant use of one of
  /// the class's generic parameters, and therefore requires covariant checks.
  IncludesTypeParametersCovariantly needsCheckVisitor;

  /// Getters and methods in the class's API.  May include forwarding nodes.
  final gettersAndMethods = <Member>[];

  /// Setters in the class's API.  May include forwarding nodes.
  final setters = <Member>[];

  ClassInferenceInfo(this.builder);
}

/// Concrete shadow object representing a set of invocation arguments.
class ArgumentsJudgment extends Arguments {
  bool _hasExplicitTypeArguments;

  List<ExpressionJudgment> get positionalJudgments => positional.cast();

  List<NamedExpressionJudgment> get namedJudgments => named.cast();

  ArgumentsJudgment(List<Expression> positional,
      {List<DartType> types, List<NamedExpression> named})
      : _hasExplicitTypeArguments = types != null && types.isNotEmpty,
        super(positional, types: types, named: named);

  static void setNonInferrableArgumentTypes(
      ArgumentsJudgment arguments, List<DartType> types) {
    arguments.types.clear();
    arguments.types.addAll(types);
    arguments._hasExplicitTypeArguments = true;
  }

  static void removeNonInferrableArgumentTypes(ArgumentsJudgment arguments) {
    arguments.types.clear();
    arguments._hasExplicitTypeArguments = false;
  }
}

/// Shadow object for [AsExpression].
class AsJudgment extends AsExpression implements ExpressionJudgment {
  final AsExpressionTokens tokens;

  DartType inferredType;

  AsJudgment(Expression operand, this.tokens, DartType type)
      : super(operand, type);

  ExpressionJudgment get judgment => operand;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferrer.inferExpression(judgment, const UnknownType(), false,
        isVoidAllowed: true);
    inferredType = type;
    inferrer.listener
        .asExpression(this, fileOffset, null, tokens, null, inferredType);
    return null;
  }
}

/// Concrete shadow object representing an assert initializer in kernel form.
class AssertInitializerJudgment extends AssertInitializer
    implements InitializerJudgment {
  final AssertInitializerTokens tokens;

  AssertInitializerJudgment(AssertStatement statement, this.tokens)
      : super(statement);

  AssertStatementJudgment get judgment => statement;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    inferrer.inferStatement(judgment);
    inferrer.listener.assertInitializer(this, fileOffset, tokens, null, null);
  }
}

/// Concrete shadow object representing an assertion statement in kernel form.
class AssertStatementJudgment extends AssertStatement
    implements StatementJudgment {
  final AssertStatementTokens tokens;

  AssertStatementJudgment(this.tokens, Expression condition,
      {Expression message, int conditionStartOffset, int conditionEndOffset})
      : super(condition,
            message: message,
            conditionStartOffset: conditionStartOffset,
            conditionEndOffset: conditionEndOffset);

  ExpressionJudgment get conditionJudgment => condition;

  ExpressionJudgment get messageJudgment => message;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    var conditionJudgment = this.conditionJudgment;
    var messageJudgment = this.messageJudgment;
    var expectedType = inferrer.coreTypes.boolClass.rawType;
    inferrer.inferExpression(
        conditionJudgment, expectedType, !inferrer.isTopLevel);
    inferrer.ensureAssignable(expectedType, conditionJudgment.inferredType,
        conditionJudgment, conditionJudgment.fileOffset);
    if (messageJudgment != null) {
      inferrer.inferExpression(messageJudgment, const UnknownType(), false);
    }
    inferrer.listener.assertStatement(this, fileOffset, tokens, null, null);
  }
}

/// Shadow object for [AwaitExpression].
class AwaitJudgment extends AwaitExpression implements ExpressionJudgment {
  AwaitExpressionTokens tokens;
  DartType inferredType;

  AwaitJudgment(this.tokens, Expression operand) : super(operand);

  ExpressionJudgment get judgment => operand;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    if (!inferrer.typeSchemaEnvironment.isEmptyContext(typeContext)) {
      typeContext = inferrer.wrapFutureOrType(typeContext);
    }
    var judgment = this.judgment;
    inferrer.inferExpression(judgment, typeContext, true, isVoidAllowed: true);
    inferredType =
        inferrer.typeSchemaEnvironment.unfutureType(judgment.inferredType);
    inferrer.listener
        .awaitExpression(this, fileOffset, tokens, null, inferredType);
    return null;
  }
}

/// Concrete shadow object representing a statement block in kernel form.
class BlockJudgment extends Block implements StatementJudgment {
  BlockTokens tokens;

  BlockJudgment(this.tokens, List<Statement> statements) : super(statements);

  List<Statement> get judgments => statements;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    for (var judgment in judgments) {
      inferrer.inferStatement(judgment);
    }
    inferrer.listener.block(this, fileOffset, tokens, null);
  }
}

/// Concrete shadow object representing a boolean literal in kernel form.
class BoolJudgment extends BoolLiteral implements ExpressionJudgment {
  final BoolLiteralTokens tokens;

  DartType inferredType;

  BoolJudgment(this.tokens, bool value) : super(value);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferredType = inferrer.coreTypes.boolClass.rawType;
    inferrer.listener
        .boolLiteral(this, fileOffset, tokens, value, inferredType);
    return null;
  }
}

/// Concrete shadow object representing a break statement in kernel form.
class BreakJudgment extends BreakStatement implements StatementJudgment {
  BreakStatementTokens tokens;

  BreakJudgment(this.tokens, LabeledStatement target) : super(target);

  LabeledStatementJudgment get targetJudgment => target;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    // No inference needs to be done.
    inferrer.listener.breakStatement(
        this, fileOffset, tokens, null, targetJudgment?.createBinder(inferrer));
  }
}

/// Concrete shadow object representing a continue statement in kernel form.
class ContinueJudgment extends BreakStatement implements StatementJudgment {
  ContinueStatementTokens tokens;

  ContinueJudgment(this.tokens, LabeledStatement target) : super(target);

  LabeledStatementJudgment get targetJudgment => target;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    // No inference needs to be done.
    inferrer.listener.continueStatement(
        this, fileOffset, tokens, null, targetJudgment?.createBinder(inferrer));
  }
}

/// Concrete shadow object representing a cascade expression.
///
/// A cascade expression of the form `a..b()..c()` is represented as the kernel
/// expression:
///
///     let v = a in
///         let _ = v.b() in
///             let _ = v.c() in
///                 v
///
/// In the documentation that follows, `v` is referred to as the "cascade
/// variable"--this is the variable that remembers the value of the expression
/// preceding the first `..` while the cascades are being evaluated.
///
/// After constructing a [CascadeJudgment], the caller should
/// call [finalize] with an expression representing the expression after the
/// `..`.  If a further `..` follows that expression, the caller should call
/// [extend] followed by [finalize] for each subsequent cascade.
class CascadeJudgment extends Let implements ExpressionJudgment {
  DartType inferredType;

  /// Pointer to the last "let" expression in the cascade.
  Let nextCascade;

  /// Creates a [CascadeJudgment] using [variable] as the cascade
  /// variable.  Caller is responsible for ensuring that [variable]'s
  /// initializer is the expression preceding the first `..` of the cascade
  /// expression.
  CascadeJudgment(VariableDeclarationJudgment variable)
      : super(
            variable,
            makeLet(new VariableDeclaration.forValue(new _UnfinishedCascade()),
                new VariableGet(variable))) {
    nextCascade = body;
  }

  ExpressionJudgment get targetJudgment => variable.initializer;

  Iterable<ExpressionJudgment> get cascadeJudgments sync* {
    Let section = body;
    while (true) {
      yield section.variable.initializer;
      if (section.body is! Let) break;
      section = section.body;
    }
  }

  /// Adds a new unfinalized section to the end of the cascade.  Should be
  /// called after the previous cascade section has been finalized.
  void extend() {
    assert(nextCascade.variable.initializer is! _UnfinishedCascade);
    Let newCascade = makeLet(
        new VariableDeclaration.forValue(new _UnfinishedCascade()),
        nextCascade.body);
    nextCascade.body = newCascade;
    newCascade.parent = nextCascade;
    nextCascade = newCascade;
  }

  /// Finalizes the last cascade section with the given [expression].
  void finalize(Expression expression) {
    assert(nextCascade.variable.initializer is _UnfinishedCascade);
    nextCascade.variable.initializer = expression;
    expression.parent = nextCascade.variable;
  }

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferredType = inferrer.inferExpression(targetJudgment, typeContext, true);
    if (inferrer.strongMode) {
      variable.type = inferredType;
    }
    for (var judgment in cascadeJudgments) {
      inferrer.inferExpression(judgment, const UnknownType(), false,
          isVoidAllowed: true);
    }
    inferrer.listener.cascadeExpression(this, fileOffset, inferredType);
    return null;
  }
}

/// Shadow object representing a class in kernel form.
class ShadowClass extends Class {
  ClassInferenceInfo _inferenceInfo;

  ShadowClass(
      {String name,
      Supertype supertype,
      Supertype mixedInType,
      List<TypeParameter> typeParameters,
      List<Supertype> implementedTypes,
      List<Procedure> procedures,
      List<Field> fields})
      : super(
            name: name,
            supertype: supertype,
            mixedInType: mixedInType,
            typeParameters: typeParameters,
            implementedTypes: implementedTypes,
            procedures: procedures,
            fields: fields);

  /// Resolves all forwarding nodes for this class, propagates covariance
  /// annotations, and creates forwarding stubs as needed.
  void finalizeCovariance(InterfaceResolver interfaceResolver) {
    interfaceResolver.finalizeCovariance(
        this, _inferenceInfo.gettersAndMethods, _inferenceInfo.builder.library);
    interfaceResolver.finalizeCovariance(
        this, _inferenceInfo.setters, _inferenceInfo.builder.library);
    interfaceResolver.recordInstrumentation(this);
  }

  /// Creates API members for this class.
  void setupApiMembers(InterfaceResolver interfaceResolver) {
    interfaceResolver.createApiMembers(this, _inferenceInfo.gettersAndMethods,
        _inferenceInfo.setters, _inferenceInfo.builder.library);
  }

  static void clearClassInferenceInfo(ShadowClass class_) {
    class_._inferenceInfo = null;
  }

  static ClassInferenceInfo getClassInferenceInfo(Class class_) {
    if (class_ is ShadowClass) return class_._inferenceInfo;
    return null;
  }

  /// Initializes the class inference information associated with the given
  /// [class_], starting with the fact that it is associated with the given
  /// [builder].
  static void setBuilder(ShadowClass class_, SourceClassBuilder builder) {
    class_._inferenceInfo = new ClassInferenceInfo(builder);
  }
}

/// Abstract shadow object representing a complex assignment in kernel form.
///
/// Since there are many forms a complex assignment might have been desugared
/// to, this class wraps the desugared assignment rather than extending it.
///
/// TODO(paulberry): once we know exactly what constitutes a "complex
/// assignment", document it here.
abstract class ComplexAssignmentJudgment extends SyntheticExpressionJudgment {
  /// In a compound assignment, the expression that reads the old value, or
  /// `null` if this is not a compound assignment.
  Expression read;

  /// The expression appearing on the RHS of the assignment.
  final ExpressionJudgment rhs;

  /// The expression that performs the write (e.g. `a.[]=(b, a.[](b) + 1)` in
  /// `++a[b]`).
  Expression write;

  /// In a compound assignment without shortcut semantics, the expression that
  /// combines the old and new values, or `null` if this is not a compound
  /// assignment.
  ///
  /// Note that in a compound assignment with shortcut semantics, this is not
  /// used; [nullAwareCombiner] is used instead.
  MethodInvocation combiner;

  /// In a compound assignment with shortcut semantics, the conditional
  /// expression that determines whether the assignment occurs.
  ///
  /// Note that in a compound assignment without shortcut semantics, this is not
  /// used; [combiner] is used instead.
  ConditionalExpression nullAwareCombiner;

  /// Indicates whether the expression arose from a post-increment or
  /// post-decrement.
  bool isPostIncDec = false;

  /// Indicates whether the expression arose from a pre-increment or
  /// pre-decrement.
  bool isPreIncDec = false;

  ComplexAssignmentJudgment(this.rhs) : super(null);

  String toString() {
    var parts = _getToStringParts();
    return '${runtimeType}(${parts.join(', ')})';
  }

  List<String> _getToStringParts() {
    List<String> parts = [];
    if (desugared != null) parts.add('desugared=$desugared');
    if (read != null) parts.add('read=$read');
    if (rhs != null) parts.add('rhs=$rhs');
    if (write != null) parts.add('write=$write');
    if (combiner != null) parts.add('combiner=$combiner');
    if (nullAwareCombiner != null) {
      parts.add('nullAwareCombiner=$nullAwareCombiner');
    }
    if (isPostIncDec) parts.add('isPostIncDec=true');
    if (isPreIncDec) parts.add('isPreIncDec=true');
    return parts;
  }

  DartType _getWriteType(ShadowTypeInferrer inferrer) => unhandled(
      '$runtimeType', 'ShadowComplexAssignment._getWriteType', -1, null);

  _ComplexAssignmentInferenceResult
      _inferRhs<Expression, Statement, Initializer, Type>(
          ShadowTypeInferrer inferrer,
          DartType readType,
          DartType writeContext) {
    assert(writeContext != null);
    if (readType is VoidType &&
        (combiner != null || nullAwareCombiner != null)) {
      inferrer.helper
          ?.addProblem(messageVoidExpression, read.fileOffset, noLength);
    }
    var writeOffset = write == null ? -1 : write.fileOffset;
    Procedure combinerMember;
    DartType combinedType;
    if (combiner != null) {
      bool isOverloadedArithmeticOperator = false;
      combinerMember =
          inferrer.findMethodInvocationMember(readType, combiner, silent: true);
      if (combinerMember is Procedure) {
        isOverloadedArithmeticOperator = inferrer.typeSchemaEnvironment
            .isOverloadedArithmeticOperatorAndType(combinerMember, readType);
      }
      DartType rhsType;
      var combinerType = inferrer.getCalleeFunctionType(
          inferrer.getCalleeType(combinerMember, readType), false);
      if (isPreIncDec || isPostIncDec) {
        rhsType = inferrer.coreTypes.intClass.rawType;
      } else {
        // It's not necessary to call _storeLetType for [rhs] because the RHS
        // is always passed directly to the combiner; it's never stored in a
        // temporary variable first.
        assert(identical(combiner.arguments.positional.first, rhs));
        // Analyzer uses a null context for the RHS here.
        // TODO(paulberry): improve on this.
        inferrer.inferExpression(rhs, const UnknownType(), true);
        rhsType = rhs.inferredType;
        // Do not use rhs after this point because it may be a Shadow node
        // that has been replaced in the tree with its desugaring.
        var expectedType = getPositionalParameterType(combinerType, 0);
        inferrer.ensureAssignable(expectedType, rhsType,
            combiner.arguments.positional.first, combiner.fileOffset);
      }
      if (isOverloadedArithmeticOperator) {
        combinedType = inferrer.typeSchemaEnvironment
            .getTypeOfOverloadedArithmetic(readType, rhsType);
      } else {
        combinedType = combinerType.returnType;
      }
      var checkKind = inferrer.preCheckInvocationContravariance(read, readType,
          combinerMember, combiner, combiner.arguments, combiner);
      var replacedCombiner = inferrer.handleInvocationContravariance(
          checkKind,
          combiner,
          combiner.arguments,
          combiner,
          combinedType,
          combinerType,
          combiner.fileOffset);
      var replacedCombiner2 = inferrer.ensureAssignable(
          writeContext, combinedType, replacedCombiner, writeOffset);
      if (replacedCombiner2 != null) {
        replacedCombiner = replacedCombiner2;
      }
      _storeLetType(inferrer, replacedCombiner, combinedType);
    } else {
      inferrer.inferExpression(rhs, writeContext ?? const UnknownType(), true,
          isVoidAllowed: true);
      var rhsType = rhs.inferredType;
      var replacedRhs = inferrer.ensureAssignable(
          writeContext, rhsType, rhs, writeOffset,
          isVoidAllowed: writeContext is VoidType);
      _storeLetType(inferrer, replacedRhs ?? rhs, rhsType);
      if (nullAwareCombiner != null) {
        MethodInvocation equalsInvocation = nullAwareCombiner.condition;
        inferrer.findMethodInvocationMember(
            greatestClosure(inferrer.coreTypes, writeContext), equalsInvocation,
            silent: true);
        // Note: the case of readType=null only happens for erroneous code.
        combinedType = readType == null
            ? rhsType
            : inferrer.typeSchemaEnvironment
                .getStandardUpperBound(readType, rhsType);
        if (inferrer.strongMode) {
          nullAwareCombiner.staticType = combinedType;
        }
      } else {
        combinedType = rhsType;
      }
    }
    if (this is IndexAssignmentJudgment) {
      _storeLetType(inferrer, write, const VoidType());
    } else {
      _storeLetType(inferrer, write, combinedType);
    }
    inferredType =
        isPostIncDec ? (readType ?? const DynamicType()) : combinedType;
    return new _ComplexAssignmentInferenceResult(combinerMember);
  }
}

/// Abstract shadow object representing a complex assignment involving a
/// receiver.
abstract class ComplexAssignmentJudgmentWithReceiver
    extends ComplexAssignmentJudgment {
  /// The receiver of the assignment target (e.g. `a` in `a[b] = c`).
  final ExpressionJudgment receiver;

  /// Indicates whether this assignment uses `super`.
  final bool isSuper;

  ComplexAssignmentJudgmentWithReceiver(
      this.receiver, ExpressionJudgment rhs, this.isSuper)
      : super(rhs);

  @override
  List<String> _getToStringParts() {
    var parts = super._getToStringParts();
    if (receiver != null) parts.add('receiver=$receiver');
    if (isSuper) parts.add('isSuper=true');
    return parts;
  }

  DartType _inferReceiver<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    if (receiver != null) {
      inferrer.inferExpression(receiver, const UnknownType(), true);
      var receiverType = receiver.inferredType;
      _storeLetType(inferrer, receiver, receiverType);
      return receiverType;
    } else if (isSuper) {
      return inferrer.classHierarchy.getTypeAsInstanceOf(
          inferrer.thisType, inferrer.thisType.classNode.supertype.classNode);
    } else {
      return inferrer.thisType;
    }
  }
}

/// Concrete shadow object representing a conditional expression in kernel form.
/// Shadow object for [ConditionalExpression].
class ConditionalJudgment extends ConditionalExpression
    implements ExpressionJudgment {
  ConditionalExpressionTokens tokens;
  DartType inferredType;

  ExpressionJudgment get conditionJudgment => condition;

  ExpressionJudgment get thenJudgment => then;

  ExpressionJudgment get otherwiseJudgment => otherwise;

  ConditionalJudgment(
      Expression condition, this.tokens, Expression then, Expression otherwise)
      : super(condition, then, otherwise, null);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var conditionJudgment = this.conditionJudgment;
    var thenJudgment = this.thenJudgment;
    var otherwiseJudgment = this.otherwiseJudgment;
    var expectedType = inferrer.coreTypes.boolClass.rawType;
    inferrer.inferExpression(
        conditionJudgment, expectedType, !inferrer.isTopLevel);
    inferrer.ensureAssignable(expectedType, conditionJudgment.inferredType,
        condition, condition.fileOffset);
    inferrer.inferExpression(thenJudgment, typeContext, true,
        isVoidAllowed: true);
    bool useLub = _forceLub || typeContext == null;
    inferrer.inferExpression(otherwiseJudgment, typeContext, useLub,
        isVoidAllowed: true);
    inferredType = useLub
        ? inferrer.typeSchemaEnvironment.getStandardUpperBound(
            thenJudgment.inferredType, otherwiseJudgment.inferredType)
        : greatestClosure(inferrer.coreTypes, typeContext);
    if (inferrer.strongMode) {
      staticType = inferredType;
    }
    inferrer.listener.conditionalExpression(
        this, fileOffset, null, tokens, null, null, inferredType);
    return null;
  }
}

/// Shadow object for [ConstructorInvocation].
class ConstructorInvocationJudgment extends ConstructorInvocation
    implements ExpressionJudgment {
  DartType inferredType;

  ConstructorInvocationJudgment(Constructor target, ArgumentsJudgment arguments,
      {bool isConst: false})
      : super(target, arguments, isConst: isConst);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var library = inferrer.engine.beingInferred[target];
    if (library != null) {
      // There is a cyclic dependency where inferring the types of the
      // initializing formals of a constructor required us to infer the
      // corresponding field type which required us to know the type of the
      // constructor.
      var name = target.enclosingClass.name;
      if (target.name.name != '') name += '.${target.name.name}';
      library.addProblem(
          templateCantInferTypeDueToCircularity.withArguments(name),
          target.fileOffset,
          name.length,
          target.fileUri);
      for (var declaration in target.function.positionalParameters) {
        declaration.type ??= const DynamicType();
      }
      for (var declaration in target.function.namedParameters) {
        declaration.type ??= const DynamicType();
      }
    } else if ((library = inferrer.engine.toBeInferred[target]) != null) {
      inferrer.engine.toBeInferred.remove(target);
      inferrer.engine.beingInferred[target] = library;
      for (var declaration in target.function.positionalParameters) {
        inferrer.engine.inferInitializingFormal(declaration, target);
      }
      for (var declaration in target.function.namedParameters) {
        inferrer.engine.inferInitializingFormal(declaration, target);
      }
      inferrer.engine.beingInferred.remove(target);
    }
    var inferenceResult = inferrer.inferInvocation(
        typeContext,
        fileOffset,
        target.function.functionType,
        computeConstructorReturnType(target),
        argumentJudgments,
        isConst: isConst);
    var inferredType = inferenceResult.type;
    this.inferredType = inferredType;
    if (inferrer.strongMode &&
        !inferrer.isTopLevel &&
        inferrer.typeSchemaEnvironment.isSuperBounded(inferredType)) {
      inferrer.helper.addProblem(
          templateCantUseSuperBoundedTypeForInstanceCreation
              .withArguments(inferredType),
          fileOffset,
          noLength);
    }
    inferrer.listener.constructorInvocation(
        this, argumentJudgments.fileOffset, target, inferredType);

    return null;
  }
}

/// Concrete shadow object representing a continue statement from a switch
/// statement, in kernel form.
class ContinueSwitchJudgment extends ContinueSwitchStatement
    implements StatementJudgment {
  ContinueSwitchStatementTokens tokens;

  ContinueSwitchJudgment(this.tokens, SwitchCase target) : super(target);

  SwitchCaseJudgment get targetJudgment => target;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    // No inference needs to be done.
    inferrer.listener.continueSwitchStatement(
        this, fileOffset, tokens, null, targetJudgment?.createBinder(inferrer));
  }
}

/// Shadow object representing a deferred check in kernel form.
class DeferredCheckJudgment extends Let implements ExpressionJudgment {
  DartType inferredType;

  DeferredCheckJudgment(VariableDeclaration variable, Expression body)
      : super(variable, body);

  ExpressionJudgment get judgment => body;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    // Since the variable is not used in the body we don't need to type infer
    // it.  We can just type infer the body.
    var judgment = this.judgment;
    inferrer.inferExpression(judgment, typeContext, true, isVoidAllowed: true);
    inferredType = judgment.inferredType;
    inferrer.listener.deferredCheck(this, fileOffset, inferredType);
    return null;
  }
}

/// Concrete shadow object representing a do loop in kernel form.
class DoJudgment extends DoStatement implements StatementJudgment {
  DoStatementTokens tokens;

  DoJudgment(this.tokens, Statement body, Expression condition)
      : super(body, condition);

  StatementJudgment get bodyJudgment => body;

  ExpressionJudgment get conditionJudgment => condition;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    var conditionJudgment = this.conditionJudgment;
    inferrer.inferStatement(bodyJudgment);
    var boolType = inferrer.coreTypes.boolClass.rawType;
    inferrer.inferExpression(conditionJudgment, boolType, !inferrer.isTopLevel);
    inferrer.ensureAssignable(boolType, conditionJudgment.inferredType,
        condition, condition.fileOffset);
    inferrer.listener.doStatement(this, fileOffset, tokens, null, null);
  }
}

/// Concrete shadow object representing a double literal in kernel form.
class DoubleJudgment extends DoubleLiteral implements ExpressionJudgment {
  DoubleLiteralTokens tokens;
  DartType inferredType;

  DoubleJudgment(this.tokens, double value) : super(value);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferredType = inferrer.coreTypes.doubleClass.rawType;
    inferrer.listener
        .doubleLiteral(this, fileOffset, tokens, value, inferredType);
    return null;
  }
}

/// Common base class for shadow objects representing expressions in kernel
/// form.
abstract class ExpressionJudgment implements Expression {
  DartType inferredType;

  /// Calls back to [inferrer] to perform type inference for whatever concrete
  /// type of [ExpressionJudgment] this is.
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext);
}

/// Concrete shadow object representing an empty statement in kernel form.
class EmptyStatementJudgment extends EmptyStatement
    implements StatementJudgment {
  EmptyStatementTokens tokens;

  EmptyStatementJudgment(this.tokens);

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    // No inference needs to be done.
    inferrer.listener.emptyStatement(tokens);
  }
}

/// Concrete shadow object representing an expression statement in kernel form.
class ExpressionStatementJudgment extends ExpressionStatement
    implements StatementJudgment {
  ExpressionStatementTokens tokens;

  ExpressionStatementJudgment(Expression expression, this.tokens)
      : super(expression);

  Expression get judgment => expression;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    inferrer.inferExpression(judgment, const UnknownType(), false,
        isVoidAllowed: true);
    inferrer.listener.expressionStatement(this, fileOffset, null, tokens);
  }
}

/// Shadow object for [StaticInvocation] when the procedure being invoked is a
/// factory constructor.
class FactoryConstructorInvocationJudgment extends StaticInvocation
    implements ExpressionJudgment {
  DartType inferredType;

  FactoryConstructorInvocationJudgment(
      Procedure target, ArgumentsJudgment arguments,
      {bool isConst: false})
      : super(target, arguments, isConst: isConst);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var inferenceResult = inferrer.inferInvocation(
        typeContext,
        fileOffset,
        target.function.functionType,
        computeConstructorReturnType(target),
        argumentJudgments,
        isConst: isConst);
    var inferredType = inferenceResult.type;
    this.inferredType = inferredType;
    inferrer.listener.constructorInvocation(
        this, argumentJudgments.fileOffset, target, inferredType);

    return null;
  }
}

/// Concrete shadow object representing a field in kernel form.
class ShadowField extends Field implements ShadowMember {
  @override
  InferenceNode inferenceNode;

  ShadowTypeInferrer _typeInferrer;

  final bool _isImplicitlyTyped;

  ShadowField(Name name, this._isImplicitlyTyped, {Uri fileUri})
      : super(name, fileUri: fileUri) {}

  @override
  void setInferredType(
      TypeInferenceEngine engine, Uri uri, DartType inferredType) {
    type = inferredType;
  }

  static bool hasTypeInferredFromInitializer(ShadowField field) =>
      field.inferenceNode is FieldInitializerInferenceNode;

  static bool isImplicitlyTyped(ShadowField field) => field._isImplicitlyTyped;

  static void setInferenceNode(ShadowField field, InferenceNode node) {
    assert(field.inferenceNode == null);
    field.inferenceNode = node;
  }
}

/// Concrete shadow object representing a field initializer in kernel form.
class ShadowFieldInitializer extends FieldInitializer
    implements InitializerJudgment {
  ShadowFieldInitializer(Field field, Expression value) : super(field, value);

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    var initializerType = inferrer.inferExpression(value, field.type, true);
    inferrer.ensureAssignable(field.type, initializerType, value, fileOffset);
    inferrer.listener.fieldInitializer(
        this, fileOffset, null, null, null, null, null, field);
  }
}

/// Concrete shadow object representing a for-in loop in kernel form.
class ForInJudgment extends ForInStatement implements StatementJudgment {
  final ForInStatementTokens tokens;

  final bool _declaresVariable;

  final SyntheticExpressionJudgment _syntheticAssignment;

  ForInJudgment(this.tokens, VariableDeclaration variable, Expression iterable,
      Statement body, this._declaresVariable, this._syntheticAssignment,
      {bool isAsync: false})
      : super(variable, iterable, body, isAsync: isAsync);

  VariableDeclarationJudgment get variableJudgment => variable;

  ExpressionJudgment get iterableJudgment => iterable;

  StatementJudgment get bodyJudgment => body;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    var iterableClass = isAsync
        ? inferrer.coreTypes.streamClass
        : inferrer.coreTypes.iterableClass;
    DartType context;
    bool typeNeeded = false;
    bool typeChecksNeeded = !inferrer.isTopLevel;
    VariableDeclarationJudgment variable;
    var syntheticAssignment = _syntheticAssignment;
    kernel.Expression syntheticWrite;
    DartType syntheticWriteType;
    if (_declaresVariable) {
      variable = this.variableJudgment;
      if (inferrer.strongMode && variable._implicitlyTyped) {
        typeNeeded = true;
        context = const UnknownType();
      } else {
        context = variable.type;
      }
    } else if (syntheticAssignment is ComplexAssignmentJudgment) {
      syntheticWrite = syntheticAssignment.write;
      syntheticWriteType =
          context = syntheticAssignment._getWriteType(inferrer);
    } else {
      context = const UnknownType();
    }
    context = inferrer.wrapType(context, iterableClass);

    var iterableJudgment = this.iterableJudgment;
    inferrer.inferExpression(
        iterableJudgment, context, typeNeeded || typeChecksNeeded);
    var inferredExpressionType =
        inferrer.resolveTypeParameter(iterableJudgment.inferredType);
    inferrer.ensureAssignable(
        inferrer.wrapType(const DynamicType(), iterableClass),
        inferredExpressionType,
        iterable,
        iterable.fileOffset,
        template: templateForInLoopTypeNotIterable);

    DartType inferredType;
    if (typeNeeded || typeChecksNeeded) {
      inferredType = const DynamicType();
      if (inferredExpressionType is InterfaceType) {
        InterfaceType supertype = inferrer.classHierarchy
            .getTypeAsInstanceOf(inferredExpressionType, iterableClass);
        if (supertype != null) {
          inferredType = supertype.typeArguments[0];
        }
      }
      if (typeNeeded) {
        inferrer.instrumentation?.record(inferrer.uri, variable.fileOffset,
            'type', new InstrumentationValueForType(inferredType));
        variable.type = inferredType;
      }
      if (!_declaresVariable) {
        this.variable.type = inferredType;
      }
    }

    inferrer.inferStatement(bodyJudgment);
    if (syntheticAssignment != null) {
      var syntheticStatement = new ExpressionStatement(syntheticAssignment);
      body = combineStatements(syntheticStatement, body)..parent = this;
    }
    if (_declaresVariable) {
      inferrer.inferMetadataKeepingHelper(variable.annotations);
      var tempVar =
          new VariableDeclaration(null, type: inferredType, isFinal: true);
      var variableGet = new VariableGet(tempVar)
        ..fileOffset = this.variable.fileOffset;
      var implicitDowncast = inferrer.ensureAssignable(
          variable.type, inferredType, variableGet, fileOffset,
          template: templateForInLoopElementTypeNotAssignable);
      if (implicitDowncast != null) {
        this.variable = tempVar..parent = this;
        variable.initializer = implicitDowncast..parent = variable;
        body = combineStatements(variable, body)..parent = this;
      }
    } else if (syntheticAssignment is SyntheticExpressionJudgment) {
      if (syntheticAssignment is ComplexAssignmentJudgment) {
        inferrer.ensureAssignable(
            greatestClosure(inferrer.coreTypes, syntheticWriteType),
            this.variable.type,
            syntheticAssignment.rhs,
            syntheticAssignment.rhs.fileOffset,
            template: templateForInLoopElementTypeNotAssignable,
            isVoidAllowed: true);
        if (syntheticAssignment is PropertyAssignmentJudgment) {
          syntheticAssignment._handleWriteContravariance(
              inferrer, inferrer.thisType);
        }
      }
      syntheticAssignment._replaceWithDesugared();
    }
    if (syntheticWrite is VariableSet) {
      inferrer.listener.forInStatement(
          this,
          fileOffset,
          tokens,
          null,
          iterable,
          body,
          variable?.createBinder(inferrer),
          variable?.type,
          syntheticWrite.fileOffset,
          syntheticWrite.variable.type,
          (syntheticWrite.variable as VariableDeclarationJudgment)
              .createBinder(inferrer),
          null);
    } else if (syntheticWrite is PropertySet) {
      inferrer.listener.forInStatement(
          this,
          fileOffset,
          tokens,
          null,
          iterable,
          body,
          variable?.createBinder(inferrer),
          variable?.type,
          syntheticWrite.fileOffset,
          syntheticWrite.interfaceTarget?.setterType,
          null,
          syntheticWrite.interfaceTarget);
    } else if (syntheticWrite is StaticSet) {
      inferrer.listener.forInStatement(
          this,
          fileOffset,
          tokens,
          null,
          iterable,
          body,
          variable?.createBinder(inferrer),
          variable?.type,
          syntheticWrite.fileOffset,
          syntheticWrite.target.setterType,
          null,
          syntheticWrite.target);
    } else if (syntheticWrite == null ||
        syntheticWrite is SyntheticExpressionJudgment) {
      inferrer.listener.forInStatement(
          this,
          fileOffset,
          tokens,
          null,
          null,
          null,
          variable?.createBinder(inferrer),
          variable?.type,
          null,
          null,
          null,
          null);
    } else {
      throw new UnimplementedError(
          '(${syntheticWrite.runtimeType}) $syntheticWrite');
    }
  }
}

/// Concrete shadow object representing a classic for loop in kernel form.
class ForJudgment extends ForStatement implements StatementJudgment {
  ForStatementTokens tokens;
  final List<ExpressionJudgment> initializers;

  ForJudgment(
      this.tokens,
      List<VariableDeclaration> variables,
      this.initializers,
      ExpressionJudgment condition,
      List<Expression> updates,
      Statement body)
      : super(variables ?? [], condition, updates, body);

  List<VariableDeclarationJudgment> get variableJudgments => variables.cast();

  ExpressionJudgment get conditionJudgment => condition;

  List<ExpressionJudgment> get updateJudgments => updates.cast();

  StatementJudgment get bodyJudgment => body;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    var initializers = this.initializers;
    var conditionJudgment = this.conditionJudgment;
    if (initializers != null) {
      for (var initializer in initializers) {
        variables
            .add(new VariableDeclaration.forValue(initializer)..parent = this);
        inferrer.inferExpression(initializer, const UnknownType(), false,
            isVoidAllowed: true);
      }
    } else {
      for (var variable in variableJudgments) {
        inferrer.inferStatement(variable);
      }
    }
    if (conditionJudgment != null) {
      var expectedType = inferrer.coreTypes.boolClass.rawType;
      inferrer.inferExpression(
          conditionJudgment, expectedType, !inferrer.isTopLevel);
      inferrer.ensureAssignable(expectedType, conditionJudgment.inferredType,
          condition, condition.fileOffset);
    }
    for (var update in updateJudgments) {
      inferrer.inferExpression(update, const UnknownType(), false,
          isVoidAllowed: true);
    }
    inferrer.inferStatement(bodyJudgment);
    inferrer.listener.forStatement(
        this, fileOffset, tokens, null, null, condition, updates, body);
  }
}

/// Concrete shadow object representing a function expression in kernel form.
class FunctionNodeJudgment extends FunctionNode {
  FunctionNodeJudgment(Statement body,
      {List<TypeParameter> typeParameters,
      List<VariableDeclaration> positionalParameters,
      List<VariableDeclaration> namedParameters,
      int requiredParameterCount,
      DartType returnType: const DynamicType(),
      AsyncMarker asyncMarker: AsyncMarker.Sync,
      AsyncMarker dartAsyncMarker})
      : super(body,
            typeParameters: typeParameters,
            positionalParameters: positionalParameters,
            namedParameters: namedParameters,
            requiredParameterCount: requiredParameterCount,
            returnType: returnType,
            asyncMarker: asyncMarker,
            dartAsyncMarker: dartAsyncMarker);

  ExpressionInferenceResult infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer,
      DartType typeContext,
      DartType returnContext,
      int returnTypeInstrumentationOffset) {
    return inferrer.inferLocalFunction(
        this, typeContext, returnTypeInstrumentationOffset, returnContext);
  }
}

/// Concrete shadow object representing a local function declaration in kernel
/// form.
class FunctionDeclarationJudgment extends FunctionDeclaration
    implements StatementJudgment {
  bool _hasImplicitReturnType = false;

  FunctionDeclarationJudgment(
      VariableDeclarationJudgment variable, FunctionNodeJudgment function)
      : super(variable, function);

  VariableDeclarationJudgment get variableJudgment => variable;

  FunctionNodeJudgment get functionJudgment => function;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    inferrer.inferMetadataKeepingHelper(variable.annotations);
    DartType returnContext = _hasImplicitReturnType
        ? (inferrer.strongMode ? null : const DynamicType())
        : function.returnType;
    var inferenceResult =
        functionJudgment.infer(inferrer, null, returnContext, fileOffset);
    var inferredType = variable.type = inferenceResult.type;
    inferrer.listener.functionDeclaration(
        variableJudgment.createBinder(inferrer), inferredType);
  }

  static void setHasImplicitReturnType(
      FunctionDeclarationJudgment declaration, bool hasImplicitReturnType) {
    declaration._hasImplicitReturnType = hasImplicitReturnType;
  }
}

/// Concrete shadow object representing a function expression in kernel form.
class FunctionExpressionJudgment extends FunctionExpression
    implements ExpressionJudgment {
  DartType inferredType;

  FunctionExpressionJudgment(FunctionNodeJudgment function) : super(function);

  FunctionNodeJudgment get judgment => function;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var judgment = this.judgment;
    var inferenceResult =
        judgment.infer(inferrer, typeContext, null, fileOffset);
    inferredType = inferenceResult.type;
    inferrer.listener.functionExpression(this, fileOffset, inferredType);
    return null;
  }
}

/// Concrete shadow object representing a super initializer in kernel form.
class InvalidSuperInitializerJudgment extends LocalInitializer
    implements InitializerJudgment {
  final Constructor target;
  final ArgumentsJudgment argumentsJudgment;

  InvalidSuperInitializerJudgment(
      this.target, this.argumentsJudgment, VariableDeclaration variable)
      : super(variable);

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    var substitution = Substitution.fromSupertype(inferrer.classHierarchy
        .getClassAsInstanceOf(
            inferrer.thisType.classNode, target.enclosingClass));
    inferrer.inferInvocation(
        null,
        fileOffset,
        substitution
            .substituteType(target.function.functionType.withoutTypeParameters),
        inferrer.thisType,
        argumentsJudgment,
        skipTypeArgumentInference: true);
    inferrer.listener.superInitializer(this, fileOffset, null, null);
  }
}

/// Concrete shadow object representing an if-null expression.
///
/// An if-null expression of the form `a ?? b` is represented as the kernel
/// expression:
///
///     let v = a in v == null ? b : v
class IfNullJudgment extends Let implements ExpressionJudgment {
  final IfNullTokens tokens;

  DartType inferredType;

  IfNullJudgment(VariableDeclaration variable, this.tokens, Expression body)
      : super(variable, body);

  @override
  ConditionalExpression get body => super.body;

  /// Returns the expression to the left of `??`.
  ExpressionJudgment get leftJudgment => variable.initializer;

  /// Returns the expression to the right of `??`.
  ExpressionJudgment get rightJudgment => body.then;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var leftJudgment = this.leftJudgment;
    var rightJudgment = this.rightJudgment;
    // To infer `e0 ?? e1` in context K:
    // - Infer e0 in context K to get T0
    inferrer.inferExpression(leftJudgment, typeContext, true);
    var lhsType = leftJudgment.inferredType;
    if (inferrer.strongMode) {
      variable.type = lhsType;
    }
    // - Let J = T0 if K is `?` else K.
    // - Infer e1 in context J to get T1
    bool useLub = _forceLub || typeContext is UnknownType;
    if (typeContext is UnknownType) {
      inferrer.inferExpression(rightJudgment, lhsType, true,
          isVoidAllowed: true);
    } else {
      inferrer.inferExpression(rightJudgment, typeContext, _forceLub,
          isVoidAllowed: true);
    }
    var rhsType = rightJudgment.inferredType;
    // - Let T = greatest closure of K with respect to `?` if K is not `_`, else
    //   UP(t0, t1)
    // - Then the inferred type is T.
    inferredType = useLub
        ? inferrer.typeSchemaEnvironment.getStandardUpperBound(lhsType, rhsType)
        : greatestClosure(inferrer.coreTypes, typeContext);
    if (inferrer.strongMode) {
      body.staticType = inferredType;
    }
    inferrer.listener
        .ifNull(this, fileOffset, null, tokens, null, inferredType);
    return null;
  }
}

/// Concrete shadow object representing an if statement in kernel form.
class IfJudgment extends IfStatement implements StatementJudgment {
  IfStatementTokens tokens;

  IfJudgment(
      this.tokens, Expression condition, Statement then, Statement otherwise)
      : super(condition, then, otherwise);

  ExpressionJudgment get conditionJudgment => condition;

  StatementJudgment get thenJudgment => then;

  StatementJudgment get otherwiseJudgment => otherwise;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    var conditionJudgment = this.conditionJudgment;
    var expectedType = inferrer.coreTypes.boolClass.rawType;
    inferrer.inferExpression(
        conditionJudgment, expectedType, !inferrer.isTopLevel);
    inferrer.ensureAssignable(expectedType, conditionJudgment.inferredType,
        condition, condition.fileOffset);
    inferrer.inferStatement(thenJudgment);
    if (otherwiseJudgment != null) {
      inferrer.inferStatement(otherwiseJudgment);
    }
    inferrer.listener.ifStatement(this, fileOffset, tokens, null, null, null);
  }
}

/// Concrete shadow object representing an assignment to a target for which
/// assignment is not allowed.
class IllegalAssignmentJudgment extends ComplexAssignmentJudgment {
  /// The offset at which the invalid assignment should be stored.
  /// If `-1`, then there is no separate location for invalid assignment.
  final int assignmentOffset;

  IllegalAssignmentJudgment(ExpressionJudgment rhs, {this.assignmentOffset: -1})
      : super(rhs) {
    rhs.parent = this;
  }

  @override
  DartType _getWriteType(ShadowTypeInferrer inferrer) {
    return const UnknownType();
  }

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    if (write != null) {
      inferrer.inferExpression(write, const UnknownType(), false);
    }
    if (assignmentOffset != -1) {
      inferrer.listener.invalidAssignment(this, assignmentOffset);
    }
    inferrer.inferExpression(rhs, const UnknownType(), false);
    _replaceWithDesugared();
    inferredType = const DynamicType();
    return null;
  }
}

/// Concrete shadow object representing an assignment to a target of the form
/// `a[b]`.
class IndexAssignmentJudgment extends ComplexAssignmentJudgmentWithReceiver {
  /// In an assignment to an index expression, the index expression.
  final ExpressionJudgment index;

  IndexAssignmentJudgment(
      ExpressionJudgment receiver, this.index, ExpressionJudgment rhs,
      {bool isSuper: false})
      : super(receiver, rhs, isSuper);

  Arguments _getInvocationArguments(
      ShadowTypeInferrer inferrer, Expression invocation) {
    if (invocation is MethodInvocation) {
      return invocation.arguments;
    } else if (invocation is SuperMethodInvocation) {
      return invocation.arguments;
    } else {
      throw unhandled("${invocation.runtimeType}", "_getInvocationArguments",
          fileOffset, inferrer.uri);
    }
  }

  @override
  List<String> _getToStringParts() {
    var parts = super._getToStringParts();
    if (index != null) parts.add('index=$index');
    return parts;
  }

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var receiverType = _inferReceiver(inferrer);
    var writeMember = inferrer.findMethodInvocationMember(receiverType, write);
    // To replicate analyzer behavior, we base type inference on the write
    // member.  TODO(paulberry): would it be better to use the read member
    // when doing compound assignment?
    var calleeType = inferrer.getCalleeFunctionType(
        inferrer.getCalleeType(writeMember, receiverType), false);
    DartType expectedIndexTypeForWrite;
    DartType indexContext = const UnknownType();
    DartType writeContext = const UnknownType();
    if (calleeType.positionalParameters.length >= 2) {
      // TODO(paulberry): we ought to get a context for the index expression
      // from the index formal parameter, but analyzer doesn't so for now we
      // replicate its behavior.
      expectedIndexTypeForWrite = calleeType.positionalParameters[0];
      writeContext = calleeType.positionalParameters[1];
    }
    inferrer.inferExpression(index, indexContext, true);
    var indexType = index.inferredType;
    _storeLetType(inferrer, index, indexType);
    if (writeContext is! UnknownType) {
      inferrer.ensureAssignable(
          expectedIndexTypeForWrite,
          indexType,
          _getInvocationArguments(inferrer, write).positional[0],
          write.fileOffset);
    }
    InvocationExpression read = this.read;
    DartType readType;
    if (read != null) {
      var readMember =
          inferrer.findMethodInvocationMember(receiverType, read, silent: true);
      var calleeFunctionType = inferrer.getCalleeFunctionType(
          inferrer.getCalleeType(readMember, receiverType), false);
      inferrer.ensureAssignable(
          getPositionalParameterType(calleeFunctionType, 0),
          indexType,
          _getInvocationArguments(inferrer, read).positional[0],
          read.fileOffset);
      readType = calleeFunctionType.returnType;
      var desugaredInvocation = read is MethodInvocation ? read : null;
      var checkKind = inferrer.preCheckInvocationContravariance(receiver,
          receiverType, readMember, desugaredInvocation, read.arguments, read);
      var replacedRead = inferrer.handleInvocationContravariance(
          checkKind,
          desugaredInvocation,
          read.arguments,
          read,
          readType,
          calleeFunctionType,
          read.fileOffset);
      _storeLetType(inferrer, replacedRead, readType);
    }
    var inferredResult = _inferRhs(inferrer, readType, writeContext);
    inferrer.listener.indexAssign(this, write.fileOffset, writeMember,
        inferredResult.combiner, inferredType);
    _replaceWithDesugared();
    return null;
  }
}

/// Common base class for shadow objects representing initializers in kernel
/// form.
abstract class InitializerJudgment implements Initializer {
  /// Performs type inference for whatever concrete type of [InitializerJudgment]
  /// this is.
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer);
}

/// Concrete shadow object representing an integer literal in kernel form.
class IntJudgment extends IntLiteral implements ExpressionJudgment {
  IntLiteralTokens tokens;

  DartType inferredType;

  IntJudgment(this.tokens, int value) : super(value);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferredType = inferrer.coreTypes.intClass.rawType;
    inferrer.listener.intLiteral(this, fileOffset, tokens, value, inferredType);
    return null;
  }
}

/// Concrete shadow object representing an invalid initializer in kernel form.
class ShadowInvalidInitializer extends LocalInitializer
    implements InitializerJudgment {
  ShadowInvalidInitializer(VariableDeclaration variable) : super(variable);

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    inferrer.inferExpression(variable.initializer, const UnknownType(), false);
    inferrer.listener.invalidInitializer(this, fileOffset);
  }
}

/// Concrete shadow object representing an invalid initializer in kernel form.
class ShadowInvalidFieldInitializer extends LocalInitializer
    implements InitializerJudgment {
  final Field field;
  final Expression value;

  ShadowInvalidFieldInitializer(
      this.field, this.value, VariableDeclaration variable)
      : super(variable) {
    value?.parent = this;
  }

  ExpressionJudgment get judgment => value;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    inferrer.inferExpression(value, field.type, false);
    inferrer.listener.fieldInitializer(
        this, fileOffset, null, null, null, null, null, field);
  }
}

/// Concrete shadow object representing a non-inverted "is" test in kernel form.
class IsJudgment extends IsExpression implements ExpressionJudgment {
  IsExpressionTokens tokens;

  DartType inferredType;

  ExpressionJudgment get judgment => operand;

  IsJudgment(Expression operand, this.tokens, DartType type)
      : super(operand, type);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferrer.inferExpression(judgment, const UnknownType(), false);
    inferredType = inferrer.coreTypes.boolClass.rawType;
    inferrer.listener
        .isExpression(this, fileOffset, null, tokens, null, inferredType);
    return null;
  }
}

/// Concrete shadow object representing an inverted "is" test in kernel form.
class IsNotJudgment extends Not implements ExpressionJudgment {
  IsNotExpressionTokens tokens;
  DartType inferredType;

  @override
  IsExpression get operand => super.operand;

  ExpressionJudgment get judgment => operand.operand;

  IsNotJudgment(Expression operand, this.tokens, DartType type, int charOffset)
      : super(new IsExpression(operand, type)..fileOffset = charOffset);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferrer.inferExpression(judgment, const UnknownType(), false);
    inferredType = inferrer.coreTypes.boolClass.rawType;
    inferrer.listener
        .isNotExpression(this, fileOffset, null, tokens, null, inferredType);
    return null;
  }
}

/// Concrete shadow object representing a labeled statement in kernel form.
class LabeledStatementJudgment extends LabeledStatement
    implements StatementJudgment {
  LabeledStatementJudgment(Statement body) : super(body);

  Object binder;

  StatementJudgment get judgment => body;

  Object createBinder(ShadowTypeInferrer inferrer) {
    // TODO(paulberry): we need one binder for each label
    return binder ??=
        inferrer.listener.binderForStatementLabel(this, fileOffset, null);
  }

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    inferrer.inferStatement(judgment);
    // TODO(paulberry): support multiple labels.
    List<Object> labels = <Object>[
      inferrer.listener.statementLabel(createBinder(inferrer), null, null)
    ];
    inferrer.listener.labeledStatement(labels, null);
  }
}

/// Type inference derivation for [LiteralList].
class ListLiteralJudgment extends ListLiteral implements ExpressionJudgment {
  ListLiteralTokens tokens;
  DartType inferredType;

  List<Expression> get judgments => expressions;

  final DartType _declaredTypeArgument;

  ListLiteralJudgment(this.tokens, List<Expression> expressions,
      {DartType typeArgument, bool isConst: false})
      : _declaredTypeArgument = typeArgument,
        super(expressions,
            typeArgument: typeArgument ?? const DynamicType(),
            isConst: isConst);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var listClass = inferrer.coreTypes.listClass;
    var listType = listClass.thisType;
    List<DartType> inferredTypes;
    DartType inferredTypeArgument;
    List<DartType> formalTypes;
    List<DartType> actualTypes;
    bool inferenceNeeded = _declaredTypeArgument == null && inferrer.strongMode;
    bool typeChecksNeeded = !inferrer.isTopLevel;
    if (inferenceNeeded || typeChecksNeeded) {
      formalTypes = [];
      actualTypes = [];
    }
    if (inferenceNeeded) {
      inferredTypes = [const UnknownType()];
      inferrer.typeSchemaEnvironment.inferGenericFunctionOrType(listType,
          listClass.typeParameters, null, null, typeContext, inferredTypes,
          isConst: isConst);
      inferredTypeArgument = inferredTypes[0];
    } else {
      inferredTypeArgument = _declaredTypeArgument ?? const DynamicType();
    }
    if (inferenceNeeded || typeChecksNeeded) {
      for (int i = 0; i < judgments.length; ++i) {
        ExpressionJudgment judgment = judgments[i];
        inferrer.inferExpression(
            judgment, inferredTypeArgument, inferenceNeeded || typeChecksNeeded,
            isVoidAllowed: true);
        if (inferenceNeeded) {
          formalTypes.add(listType.typeArguments[0]);
        }
        actualTypes.add(judgment.inferredType);
      }
    }
    if (inferenceNeeded) {
      inferrer.typeSchemaEnvironment.inferGenericFunctionOrType(
          listType,
          listClass.typeParameters,
          formalTypes,
          actualTypes,
          typeContext,
          inferredTypes);
      inferredTypeArgument = inferredTypes[0];
      inferrer.instrumentation?.record(inferrer.uri, fileOffset, 'typeArgs',
          new InstrumentationValueForTypeArgs([inferredTypeArgument]));
      typeArgument = inferredTypeArgument;
    }
    if (typeChecksNeeded) {
      for (int i = 0; i < judgments.length; i++) {
        inferrer.ensureAssignable(
            typeArgument, actualTypes[i], judgments[i], judgments[i].fileOffset,
            isVoidAllowed: typeArgument is VoidType);
      }
    }
    var inferredType = new InterfaceType(listClass, [inferredTypeArgument]);
    inferrer.listener
        .listLiteral(this, fileOffset, tokens, null, expressions, inferredType);
    this.inferredType = inferredType;
    return null;
  }
}

/// Shadow object for [LogicalExpression].
class LogicalJudgment extends LogicalExpression implements ExpressionJudgment {
  LogicalExpressionTokens tokens;
  DartType inferredType;

  LogicalJudgment(
      Expression left, this.tokens, String operator, Expression right)
      : super(left, operator, right);

  ExpressionJudgment get leftJudgment => left;

  ExpressionJudgment get rightJudgment => right;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var boolType = inferrer.coreTypes.boolClass.rawType;
    var leftJudgment = this.leftJudgment;
    var rightJudgment = this.rightJudgment;
    inferrer.inferExpression(leftJudgment, boolType, !inferrer.isTopLevel);
    inferrer.inferExpression(rightJudgment, boolType, !inferrer.isTopLevel);
    inferrer.ensureAssignable(
        boolType, leftJudgment.inferredType, left, left.fileOffset);
    inferrer.ensureAssignable(
        boolType, rightJudgment.inferredType, right, right.fileOffset);
    inferredType = boolType;
    inferrer.listener
        .logicalExpression(this, fileOffset, null, tokens, null, inferredType);
    return null;
  }
}

/// Type inference derivation for [MapEntry].
///
/// This derivation is needed for uniformity.
class MapEntryJudgment extends MapEntry {
  DartType inferredKeyType;
  DartType inferredValueType;

  ExpressionJudgment get keyJudgment => key;

  ExpressionJudgment get valueJudgment => value;

  MapEntryJudgment(Expression key, Expression value) : super(key, value);

  MapEntry infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer,
      DartType keyTypeContext,
      DartType valueTypeContext) {
    ExpressionJudgment keyJudgment = this.keyJudgment;
    inferrer.inferExpression(keyJudgment, keyTypeContext, true,
        isVoidAllowed: true);
    inferredKeyType = keyJudgment.inferredType;

    ExpressionJudgment valueJudgment = this.valueJudgment;
    inferrer.inferExpression(valueJudgment, valueTypeContext, true,
        isVoidAllowed: true);
    inferredValueType = valueJudgment.inferredType;

    return null;
  }
}

/// Type inference derivation for [MapLiteral].
class MapLiteralJudgment extends MapLiteral implements ExpressionJudgment {
  MapLiteralTokens tokens;
  DartType inferredType;

  List<MapEntryJudgment> get judgments => entries;

  final DartType _declaredKeyType;
  final DartType _declaredValueType;

  MapLiteralJudgment(this.tokens, List<MapEntryJudgment> judgments,
      {DartType keyType, DartType valueType, bool isConst: false})
      : _declaredKeyType = keyType,
        _declaredValueType = valueType,
        super(judgments,
            keyType: keyType ?? const DynamicType(),
            valueType: valueType ?? const DynamicType(),
            isConst: isConst);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var mapClass = inferrer.coreTypes.mapClass;
    var mapType = mapClass.thisType;
    List<DartType> inferredTypes;
    DartType inferredKeyType;
    DartType inferredValueType;
    List<DartType> formalTypes;
    List<DartType> actualTypes;
    assert((_declaredKeyType == null) == (_declaredValueType == null));
    bool inferenceNeeded = _declaredKeyType == null && inferrer.strongMode;
    bool typeChecksNeeded = !inferrer.isTopLevel;
    if (inferenceNeeded || typeChecksNeeded) {
      formalTypes = [];
      actualTypes = [];
    }
    if (inferenceNeeded) {
      inferredTypes = [const UnknownType(), const UnknownType()];
      inferrer.typeSchemaEnvironment.inferGenericFunctionOrType(mapType,
          mapClass.typeParameters, null, null, typeContext, inferredTypes,
          isConst: isConst);
      inferredKeyType = inferredTypes[0];
      inferredValueType = inferredTypes[1];
    } else {
      inferredKeyType = _declaredKeyType ?? const DynamicType();
      inferredValueType = _declaredValueType ?? const DynamicType();
    }
    List<ExpressionJudgment> cachedKeyJudgments =
        judgments.map((j) => j.keyJudgment).toList();
    List<ExpressionJudgment> cachedValueJudgments =
        judgments.map((j) => j.valueJudgment).toList();
    if (inferenceNeeded || typeChecksNeeded) {
      for (MapEntryJudgment judgment in judgments) {
        judgment.infer(inferrer, inferredKeyType, inferredValueType);
        if (inferenceNeeded) {
          formalTypes.addAll(mapType.typeArguments);
        }
        actualTypes.add(judgment.inferredKeyType);
        actualTypes.add(judgment.inferredValueType);
      }
    }
    if (inferenceNeeded) {
      inferrer.typeSchemaEnvironment.inferGenericFunctionOrType(
          mapType,
          mapClass.typeParameters,
          formalTypes,
          actualTypes,
          typeContext,
          inferredTypes);
      inferredKeyType = inferredTypes[0];
      inferredValueType = inferredTypes[1];
      inferrer.instrumentation?.record(
          inferrer.uri,
          fileOffset,
          'typeArgs',
          new InstrumentationValueForTypeArgs(
              [inferredKeyType, inferredValueType]));
      keyType = inferredKeyType;
      valueType = inferredValueType;
    }
    if (typeChecksNeeded) {
      for (int i = 0; i < judgments.length; ++i) {
        ExpressionJudgment keyJudgment = cachedKeyJudgments[i];
        inferrer.ensureAssignable(
            keyType, actualTypes[2 * i], keyJudgment, keyJudgment.fileOffset,
            isVoidAllowed: keyType is VoidType);

        ExpressionJudgment valueJudgment = cachedValueJudgments[i];
        inferrer.ensureAssignable(valueType, actualTypes[2 * i + 1],
            valueJudgment, valueJudgment.fileOffset,
            isVoidAllowed: valueType is VoidType);
      }
    }
    inferredType =
        new InterfaceType(mapClass, [inferredKeyType, inferredValueType]);
    inferrer.listener
        .mapLiteral(this, fileOffset, tokens, null, entries, inferredType);
    return null;
  }
}

/// Abstract shadow object representing a field or procedure in kernel form.
abstract class ShadowMember implements Member {
  Uri get fileUri;

  InferenceNode get inferenceNode;

  void set inferenceNode(InferenceNode value);

  void setInferredType(
      TypeInferenceEngine engine, Uri uri, DartType inferredType);

  static void resolveInferenceNode(Member member) {
    if (member is ShadowMember) {
      if (member.inferenceNode != null) {
        member.inferenceNode.resolve();
        member.inferenceNode = null;
      }
    }
  }
}

/// Shadow object for [MethodInvocation].
class MethodInvocationJudgment extends MethodInvocation
    implements ExpressionJudgment {
  final kernel.Expression desugaredError;
  DartType inferredType;

  /// Indicates whether this method invocation is a call to a `call` method
  /// resulting from the invocation of a function expression.
  final bool _isImplicitCall;

  MethodInvocationJudgment(
      Expression receiver, Name name, ArgumentsJudgment arguments,
      {this.desugaredError, bool isImplicitCall: false, Member interfaceTarget})
      : _isImplicitCall = isImplicitCall,
        super(receiver, name, arguments, interfaceTarget);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var inferenceResult = inferrer.inferMethodInvocation(
        this, receiver, fileOffset, _isImplicitCall, typeContext,
        desugaredInvocation: this);
    inferredType = inferenceResult.type;
    if (desugaredError != null) {
      parent.replaceChild(this, desugaredError);
      parent = null;
    }
    return null;
  }
}

/// Concrete shadow object representing a named function expression.
///
/// Named function expressions are not legal in Dart, but they are accepted by
/// the parser and BodyBuilder for error recovery purposes.
///
/// A named function expression of the form `f() { ... }` is represented as the
/// kernel expression:
///
///     let f = () { ... } in f
class NamedFunctionExpressionJudgment extends Let
    implements ExpressionJudgment {
  DartType inferredType;

  NamedFunctionExpressionJudgment(VariableDeclarationJudgment variable)
      : super(variable, new VariableGet(variable));

  VariableDeclarationJudgment get variableJudgment => variable;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    ExpressionJudgment initializer = variableJudgment.initializer;
    inferrer.inferExpression(initializer, typeContext, true);
    inferredType = initializer.inferredType;
    if (inferrer.strongMode) variable.type = inferredType;
    inferrer.listener.namedFunctionExpression(this, fileOffset, inferredType);
    return null;
  }
}

/// Shadow object for [Not].
class NotJudgment extends Not implements ExpressionJudgment {
  final bool isSynthetic;
  final NotTokens tokens;

  DartType inferredType;

  NotJudgment(this.isSynthetic, this.tokens, ExpressionJudgment operand)
      : super(operand);

  ExpressionJudgment get judgment => operand;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var judgment = this.judgment;
    // First infer the receiver so we can look up the method that was invoked.
    var boolType = inferrer.coreTypes.boolClass.rawType;
    inferrer.inferExpression(judgment, boolType, !inferrer.isTopLevel);
    inferrer.ensureAssignable(
        boolType, judgment.inferredType, operand, fileOffset);
    inferredType = boolType;
    // TODO(scheglov) Temporary: https://github.com/dart-lang/sdk/issues/33666
    if (!isSynthetic) {
      inferrer.listener.not(this, fileOffset, tokens, null, inferredType);
    }
    return null;
  }
}

/// Concrete shadow object representing a null-aware method invocation.
///
/// A null-aware method invocation of the form `a?.b(...)` is represented as the
/// expression:
///
///     let v = a in v == null ? null : v.b(...)
class NullAwareMethodInvocationJudgment extends Let
    implements ExpressionJudgment {
  final kernel.Expression desugaredError;
  DartType inferredType;

  NullAwareMethodInvocationJudgment(
      VariableDeclaration variable, Expression body,
      {this.desugaredError})
      : super(variable, body);

  @override
  ConditionalExpression get body => super.body;

  MethodInvocation get _desugaredInvocation => body.otherwise;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var inferenceResult = inferrer.inferMethodInvocation(
        this, variable.initializer, fileOffset, false, typeContext,
        receiverVariable: variable, desugaredInvocation: _desugaredInvocation);
    inferredType = inferenceResult.type;
    if (inferrer.strongMode) {
      body.staticType = inferredType;
    }
    return null;
  }
}

/// Concrete shadow object representing a null-aware read from a property.
///
/// A null-aware property get of the form `a?.b` is represented as the kernel
/// expression:
///
///     let v = a in v == null ? null : v.b
class NullAwarePropertyGetJudgment extends Let implements ExpressionJudgment {
  DartType inferredType;

  NullAwarePropertyGetJudgment(
      VariableDeclaration variable, ConditionalExpression body)
      : super(variable, body);

  @override
  ConditionalExpression get body => super.body;

  PropertyGet get _desugaredGet => body.otherwise;

  ExpressionJudgment get receiverJudgment => variable.initializer;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferrer.inferPropertyGet(
        this, receiverJudgment, fileOffset, false, typeContext,
        receiverVariable: variable, desugaredGet: _desugaredGet);
    if (inferrer.strongMode) {
      body.staticType = inferredType;
    }
    return null;
  }
}

/// Concrete shadow object representing a null literal in kernel form.
class NullJudgment extends NullLiteral implements ExpressionJudgment {
  NullLiteralTokens tokens;

  DartType inferredType;

  NullJudgment(this.tokens);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferredType = inferrer.coreTypes.nullClass.rawType;
    inferrer.listener
        .nullLiteral(this, fileOffset, tokens, fileOffset == -1, inferredType);
    return null;
  }
}

/// Concrete shadow object representing a procedure in kernel form.
class ShadowProcedure extends Procedure implements ShadowMember {
  @override
  InferenceNode inferenceNode;

  final bool _hasImplicitReturnType;

  ShadowProcedure(Name name, ProcedureKind kind, FunctionNode function,
      this._hasImplicitReturnType,
      {Uri fileUri, bool isAbstract: false})
      : super(name, kind, function, fileUri: fileUri, isAbstract: isAbstract);

  @override
  void setInferredType(
      TypeInferenceEngine engine, Uri uri, DartType inferredType) {
    if (isSetter) {
      if (function.positionalParameters.length > 0) {
        function.positionalParameters[0].type = inferredType;
      }
    } else if (isGetter) {
      function.returnType = inferredType;
    } else {
      unhandled("setInferredType", "not accessor", fileOffset, uri);
    }
  }

  static bool hasImplicitReturnType(ShadowProcedure procedure) {
    return procedure._hasImplicitReturnType;
  }
}

/// Concrete shadow object representing an assignment to a property.
class PropertyAssignmentJudgment extends ComplexAssignmentJudgmentWithReceiver {
  /// If this assignment uses null-aware access (`?.`), the conditional
  /// expression that guards the access; otherwise `null`.
  ConditionalExpression nullAwareGuard;

  PropertyAssignmentJudgment(
      ExpressionJudgment receiver, ExpressionJudgment rhs,
      {bool isSuper: false})
      : super(receiver, rhs, isSuper);

  @override
  List<String> _getToStringParts() {
    var parts = super._getToStringParts();
    if (nullAwareGuard != null) parts.add('nullAwareGuard=$nullAwareGuard');
    return parts;
  }

  @override
  DartType _getWriteType(ShadowTypeInferrer inferrer) {
    assert(receiver == null);
    var receiverType = inferrer.thisType;
    var writeMember = inferrer.findPropertySetMember(receiverType, write);
    return inferrer.getSetterType(writeMember, receiverType);
  }

  Object _handleWriteContravariance(
      ShadowTypeInferrer inferrer, DartType receiverType) {
    return inferrer.findPropertySetMember(receiverType, write);
  }

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var receiverType = _inferReceiver(inferrer);

    DartType readType;
    if (read != null) {
      var readMember =
          inferrer.findPropertyGetMember(receiverType, read, silent: true);
      readType = inferrer.getCalleeType(readMember, receiverType);
      inferrer.handlePropertyGetContravariance(receiver, readMember,
          read is PropertyGet ? read : null, read, readType, read.fileOffset);
      _storeLetType(inferrer, read, readType);
    }
    Member writeMember;
    if (write != null) {
      writeMember = _handleWriteContravariance(inferrer, receiverType);
    }
    // To replicate analyzer behavior, we base type inference on the write
    // member.  TODO(paulberry): would it be better to use the read member when
    // doing compound assignment?
    var writeContext = inferrer.getSetterType(writeMember, receiverType);
    var inferredResult = _inferRhs(inferrer, readType, writeContext);
    if (inferrer.strongMode) nullAwareGuard?.staticType = inferredType;
    inferrer.listener.propertyAssign(
        this,
        write.fileOffset,
        inferrer.getRealTarget(writeMember),
        writeContext,
        inferredResult.combiner,
        inferredType);
    _replaceWithDesugared();
    return null;
  }
}

/// Shadow object for [PropertyGet].
class PropertyGetJudgment extends PropertyGet implements ExpressionJudgment {
  DartType inferredType;

  final bool forSyntheticToken;

  PropertyGetJudgment(Expression receiver, Name name,
      {Member interfaceTarget, this.forSyntheticToken = false})
      : super(receiver, name, interfaceTarget);

  PropertyGetJudgment.byReference(
      Expression receiver, Name name, Reference interfaceTargetReference)
      : forSyntheticToken = false,
        super.byReference(receiver, name, interfaceTargetReference);

  ExpressionJudgment get receiverJudgment => receiver;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferrer.inferPropertyGet(
        this, receiverJudgment, fileOffset, forSyntheticToken, typeContext,
        desugaredGet: this);
    return null;
  }
}

/// Concrete shadow object representing a redirecting initializer in kernel
/// form.
class RedirectingInitializerJudgment extends RedirectingInitializer
    implements InitializerJudgment {
  RedirectingInitializerJudgment(
      Constructor target, ArgumentsJudgment arguments)
      : super(target, arguments);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  infer<Expression, Statement, Initializer, Type>(ShadowTypeInferrer inferrer) {
    List<TypeParameter> classTypeParameters =
        target.enclosingClass.typeParameters;
    List<DartType> typeArguments =
        new List<DartType>(classTypeParameters.length);
    for (int i = 0; i < typeArguments.length; i++) {
      typeArguments[i] = new TypeParameterType(classTypeParameters[i]);
    }
    ArgumentsJudgment.setNonInferrableArgumentTypes(arguments, typeArguments);
    inferrer.inferInvocation(null, fileOffset, target.function.functionType,
        target.enclosingClass.thisType, argumentJudgments,
        skipTypeArgumentInference: true);
    ArgumentsJudgment.removeNonInferrableArgumentTypes(arguments);
    inferrer.listener.redirectingInitializer(
        this, fileOffset, null, null, null, null, target);
  }
}

/// Shadow object for [Rethrow].
class RethrowJudgment extends Rethrow implements ExpressionJudgment {
  RethrowTokens tokens;
  final kernel.Expression desugaredError;

  DartType inferredType;

  RethrowJudgment(this.tokens, this.desugaredError);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferredType = const BottomType();
    inferrer.listener.rethrow_(this, fileOffset, tokens, inferredType);
    if (desugaredError != null) {
      parent.replaceChild(this, desugaredError);
      parent = null;
    }
    return null;
  }
}

/// Concrete shadow object representing a return statement in kernel form.
class ReturnJudgment extends ReturnStatement implements StatementJudgment {
  final ReturnStatementTokens tokens;
  final String returnKeywordLexeme;

  ReturnJudgment(this.tokens, this.returnKeywordLexeme, [Expression expression])
      : super(expression);

  ExpressionJudgment get judgment => expression;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    var judgment = this.judgment;
    var closureContext = inferrer.closureContext;
    DartType typeContext = !closureContext.isGenerator
        ? closureContext.returnOrYieldContext
        : const UnknownType();
    DartType inferredType;
    if (expression != null) {
      inferrer.inferExpression(judgment, typeContext, true,
          isVoidAllowed: true);
      inferredType = judgment.inferredType;
    } else {
      inferredType = inferrer.coreTypes.nullClass.rawType;
    }
    closureContext.handleReturn(inferrer, this, inferredType,
        !identical(returnKeywordLexeme, "return"));
    inferrer.listener.returnStatement(this, fileOffset, tokens, null);
  }
}

/// Common base class for shadow objects representing statements in kernel
/// form.
abstract class StatementJudgment extends Statement {
  /// Calls back to [inferrer] to perform type inference for whatever concrete
  /// type of [StatementJudgment] this is.
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer);
}

/// Concrete shadow object representing an assignment to a static variable.
class StaticAssignmentJudgment extends ComplexAssignmentJudgment {
  StaticAssignmentJudgment(ExpressionJudgment rhs) : super(rhs);

  @override
  DartType _getWriteType(ShadowTypeInferrer inferrer) {
    StaticSet write = this.write;
    return write.target.setterType;
  }

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    DartType readType = const DynamicType(); // Only used in error recovery
    var read = this.read;
    if (read is StaticGet) {
      readType = read.target.getterType;
      _storeLetType(inferrer, read, readType);
    }
    Member writeMember;
    DartType writeContext = const UnknownType();
    var write = this.write;
    if (write is StaticSet) {
      writeContext = write.target.setterType;
      writeMember = write.target;
      if (writeMember is ShadowField && writeMember.inferenceNode != null) {
        writeMember.inferenceNode.resolve();
        writeMember.inferenceNode = null;
      }
    }
    var inferredResult = _inferRhs(inferrer, readType, writeContext);
    inferrer.listener.staticAssign(
        this,
        write?.fileOffset,
        writeMember,
        writeContext is UnknownType ? const DynamicType() : writeContext,
        inferredResult.combiner,
        inferredType);
    _replaceWithDesugared();
    return null;
  }
}

/// Concrete shadow object representing a read of a static variable in kernel
/// form.
class StaticGetJudgment extends StaticGet implements ExpressionJudgment {
  DartType inferredType;

  StaticGetJudgment(Member target) : super(target);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var target = this.target;
    if (target is ShadowField && target.inferenceNode != null) {
      target.inferenceNode.resolve();
      target.inferenceNode = null;
    }
    var type = target.getterType;
    if (target is Procedure && target.kind == ProcedureKind.Method) {
      type = inferrer.instantiateTearOff(type, typeContext, this);
    }
    inferredType = type;
    inferrer.listener.staticGet(this, fileOffset, target, inferredType);
    return null;
  }
}

/// Shadow object for [StaticInvocation].
class StaticInvocationJudgment extends StaticInvocation
    implements ExpressionJudgment {
  final kernel.Expression desugaredError;
  DartType inferredType;

  StaticInvocationJudgment(Procedure target, ArgumentsJudgment arguments,
      {this.desugaredError, bool isConst: false})
      : super(target, arguments, isConst: isConst);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    FunctionType calleeType = target != null
        ? target.function.functionType
        : new FunctionType([], const DynamicType());
    var inferenceResult = inferrer.inferInvocation(typeContext, fileOffset,
        calleeType, calleeType.returnType, argumentJudgments);
    var inferredType = inferenceResult.type;
    this.inferredType = inferredType;
    inferrer.listener.staticInvocation(
        this,
        arguments.fileOffset,
        target,
        arguments.types,
        inferrer.lastCalleeType,
        inferrer.lastInferredSubstitution,
        inferredType);
    if (desugaredError != null) {
      parent.replaceChild(this, desugaredError);
      parent = null;
    }
    return null;
  }
}

/// Concrete shadow object representing a string concatenation in kernel form.
class StringConcatenationJudgment extends StringConcatenation
    implements ExpressionJudgment {
  DartType inferredType;

  StringConcatenationJudgment(List<Expression> expressions)
      : super(expressions);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    if (!inferrer.isTopLevel) {
      for (var expression in expressions) {
        inferrer.inferExpression(expression, const UnknownType(), false);
      }
    }
    inferredType = inferrer.coreTypes.stringClass.rawType;
    inferrer.listener.stringConcatenation(this, fileOffset, inferredType);
    return null;
  }
}

/// Type inference derivation for [StringLiteral].
class StringLiteralJudgment extends StringLiteral
    implements ExpressionJudgment {
  StringLiteralTokens tokens;
  DartType inferredType;

  StringLiteralJudgment(this.tokens, String value) : super(value);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var inferredType = inferrer.coreTypes.stringClass.rawType;
    inferrer.listener
        .stringLiteral(this, fileOffset, tokens, value, inferredType);
    this.inferredType = inferredType;
    return null;
  }
}

/// Concrete shadow object representing a super initializer in kernel form.
class SuperInitializerJudgment extends SuperInitializer
    implements InitializerJudgment {
  SuperInitializerTokens tokens;

  SuperInitializerJudgment(
      this.tokens, Constructor target, ArgumentsJudgment arguments)
      : super(target, arguments);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    var substitution = Substitution.fromSupertype(inferrer.classHierarchy
        .getClassAsInstanceOf(
            inferrer.thisType.classNode, target.enclosingClass));
    inferrer.inferInvocation(
        null,
        fileOffset,
        substitution
            .substituteType(target.function.functionType.withoutTypeParameters),
        inferrer.thisType,
        argumentJudgments,
        skipTypeArgumentInference: true);
    inferrer.listener.superInitializer(this, fileOffset, tokens, null);
  }
}

/// Shadow object for [SuperMethodInvocation].
class SuperMethodInvocationJudgment extends SuperMethodInvocation
    implements ExpressionJudgment {
  final kernel.Expression desugaredError;
  DartType inferredType;

  SuperMethodInvocationJudgment(Name name, ArgumentsJudgment arguments,
      {this.desugaredError, Procedure interfaceTarget})
      : super(name, arguments, interfaceTarget);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    if (interfaceTarget != null) {
      inferrer.instrumentation?.record(inferrer.uri, fileOffset, 'target',
          new InstrumentationValueForMember(interfaceTarget));
    }
    var inferenceResult = inferrer.inferMethodInvocation(
        this, null, fileOffset, false, typeContext,
        interfaceMember: interfaceTarget,
        methodName: name,
        arguments: arguments);
    inferredType = inferenceResult.type;
    if (desugaredError != null) {
      parent.replaceChild(this, desugaredError);
      parent = null;
    }
    return null;
  }
}

/// Shadow object for [SuperPropertyGet].
class SuperPropertyGetJudgment extends SuperPropertyGet
    implements ExpressionJudgment {
  final kernel.Expression desugaredError;
  DartType inferredType;

  SuperPropertyGetJudgment(Name name,
      {this.desugaredError, Member interfaceTarget})
      : super(name, interfaceTarget);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    if (interfaceTarget != null) {
      inferrer.instrumentation?.record(inferrer.uri, fileOffset, 'target',
          new InstrumentationValueForMember(interfaceTarget));
    }
    inferrer.inferPropertyGet(this, null, fileOffset, false, typeContext,
        interfaceMember: interfaceTarget, propertyName: name);
    if (desugaredError != null) {
      parent.replaceChild(this, desugaredError);
      parent = null;
    }
    return null;
  }
}

/// Concrete shadow object representing a switch case.
class SwitchCaseJudgment extends SwitchCase {
  SwitchCaseTokens tokens;
  Object binder;

  SwitchCaseJudgment(this.tokens, List<Expression> expressions,
      List<int> expressionOffsets, Statement body,
      {bool isDefault: false})
      : super(expressions, expressionOffsets, body, isDefault: isDefault);

  SwitchCaseJudgment.defaultCase(this.tokens, Statement body)
      : super.defaultCase(body);

  SwitchCaseJudgment.empty()
      : tokens = null,
        super.empty();

  List<ExpressionJudgment> get expressionJudgments => expressions.cast();

  StatementJudgment get bodyJudgment => body;

  Object createBinder(ShadowTypeInferrer inferrer) {
    // TODO(paulberry): we need one binder for each label
    return binder ??=
        inferrer.listener.binderForSwitchLabel(this, fileOffset, null);
  }
}

/// Concrete shadow object representing a switch statement in kernel form.
class SwitchStatementJudgment extends SwitchStatement
    implements StatementJudgment {
  SwitchStatementTokens tokens;

  SwitchStatementJudgment(
      this.tokens, Expression expression, List<SwitchCase> cases)
      : super(expression, cases);

  ExpressionJudgment get expressionJudgment => expression;

  List<SwitchCaseJudgment> get caseJudgments => cases.cast();

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    var expressionJudgment = this.expressionJudgment;
    inferrer.inferExpression(expressionJudgment, const UnknownType(), true);
    var expressionType = expressionJudgment.inferredType;

    for (var switchCase in caseJudgments) {
      for (var caseExpression in switchCase.expressionJudgments) {
        DartType caseExpressionType =
            inferrer.inferExpression(caseExpression, expressionType, true);

        // Check whether the expression type is assignable to the case expression type.
        if (!inferrer.isAssignable(expressionType, caseExpressionType)) {
          inferrer.helper.addProblem(
              templateSwitchExpressionNotAssignable.withArguments(
                  expressionType, caseExpressionType),
              caseExpression.fileOffset,
              noLength,
              context: [
                messageSwitchExpressionNotAssignableCause.withLocation(
                    inferrer.uri, expressionJudgment.fileOffset, noLength)
              ]);
        }
      }
      inferrer.inferStatement(switchCase.bodyJudgment);
      // TODO(paulberry): support labels.
      inferrer.listener.switchCase(switchCase, null, null, null, null, null);
    }

    inferrer.listener
        .switchStatement(this, fileOffset, tokens, expression, cases);
  }
}

/// Shadow object for [SymbolLiteral].
class SymbolLiteralJudgment extends SymbolLiteral
    implements ExpressionJudgment {
  DartType inferredType;

  SymbolLiteralJudgment(String value) : super(value);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferredType = inferrer.coreTypes.symbolClass.rawType;
    inferrer.listener
        .symbolLiteral(this, fileOffset, null, null, null, inferredType);
    return null;
  }
}

/// Synthetic judgment class representing an attempt to invoke an unresolved
/// constructor, or a constructor that cannot be invoked, or a resolved
/// constructor with wrong number of arguments.
class InvalidConstructorInvocationJudgment extends SyntheticExpressionJudgment {
  final Member constructor;
  final Arguments arguments;

  InvalidConstructorInvocationJudgment(
      kernel.Expression desugared, this.constructor, this.arguments)
      : super(desugared);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    FunctionType calleeType;
    DartType returnType;
    if (constructor != null) {
      calleeType = constructor.function.functionType;
      returnType = computeConstructorReturnType(constructor);
    } else {
      calleeType = new FunctionType([], const DynamicType());
      returnType = const DynamicType();
    }
    ExpressionInferenceResult inferenceResult = inferrer.inferInvocation(
        typeContext, fileOffset, calleeType, returnType, argumentJudgments);
    this.inferredType = inferenceResult.type;
    inferrer.listener.constructorInvocation(
        this, arguments.fileOffset, constructor, inferredType);
    return super.infer(inferrer, typeContext);
  }
}

/// Synthetic judgment class representing an attempt to write to a read-only
/// local variable.
class InvalidVariableWriteJudgment extends SyntheticExpressionJudgment {
  /// Note: private to avoid colliding with Let.variable.
  final VariableDeclarationJudgment _variable;

  InvalidVariableWriteJudgment(kernel.Expression desugared, this._variable)
      : super(desugared);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferrer.listener.variableAssign(this, fileOffset, _variable.type,
        _variable.createBinder(inferrer), null, _variable.type);
    return super.infer(inferrer, typeContext);
  }
}

/// Synthetic judgment class representing an attempt to assign to the
/// [expression] which is not assignable.
class InvalidWriteJudgment extends SyntheticExpressionJudgment {
  final ExpressionJudgment expression;

  InvalidWriteJudgment(kernel.Expression desugared, this.expression)
      : super(desugared);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    // When a compound assignment, the expression is already wrapping in
    // VariableDeclaration in _makeRead(). Otherwise, temporary associate
    // the expression with this node.
    expression.parent ??= this;

    inferrer.inferExpression(expression, const UnknownType(), false);
    return super.infer(inferrer, typeContext);
  }
}

/// Synthetic judgment class representing an attempt reference a member
/// that is not allowed at this location.
class InvalidPropertyGetJudgment extends SyntheticExpressionJudgment {
  final Member member;

  InvalidPropertyGetJudgment(kernel.Expression desugared, this.member)
      : super(desugared);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var inferredType = member?.getterType ?? const DynamicType();
    inferrer.listener
        .propertyGet(this, fileOffset, false, member, inferredType);
    return super.infer(inferrer, typeContext);
  }
}

/// Shadow object for expressions that are introduced by the front end as part
/// of desugaring or the handling of error conditions.
///
/// These expressions are removed by type inference and replaced with their
/// desugared equivalents.
class SyntheticExpressionJudgment extends Let implements ExpressionJudgment {
  DartType inferredType;

  SyntheticExpressionJudgment(Expression desugared)
      : super(new VariableDeclaration('_', initializer: new NullLiteral()),
            desugared);

  /// The desugared kernel representation of this synthetic expression.
  Expression get desugared => body;

  void set desugared(Expression value) {
    this.body = value;
    value.parent = this;
  }

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    _replaceWithDesugared();
    inferredType = const DynamicType();
    return null;
  }

  /// Removes this expression from the expression tree, replacing it with
  /// [desugared].
  void _replaceWithDesugared() {
    parent.replaceChild(this, desugared);
    parent = null;
  }

  /// Updates any [Let] nodes in the desugared expression to account for the
  /// fact that [expression] has the given [type].
  void _storeLetType(
      TypeInferrerImpl inferrer, Expression expression, DartType type) {
    if (!inferrer.strongMode) return;
    Expression desugared = this.desugared;
    while (true) {
      if (desugared is Let) {
        Let desugaredLet = desugared;
        var variable = desugaredLet.variable;
        if (identical(variable.initializer, expression)) {
          variable.type = type;
          return;
        }
        desugared = desugaredLet.body;
      } else if (desugared is ConditionalExpression) {
        // When a null-aware assignment is desugared, often the "then" or "else"
        // branch of the conditional expression often contains "let" nodes that
        // need to be updated.
        ConditionalExpression desugaredConditionalExpression = desugared;
        if (desugaredConditionalExpression.then is Let) {
          desugared = desugaredConditionalExpression.then;
        } else {
          desugared = desugaredConditionalExpression.otherwise;
        }
      } else {
        break;
      }
    }
  }
}

class ThisJudgment extends ThisExpression implements ExpressionJudgment {
  final ThisExpressionTokens tokens;

  DartType inferredType;

  ThisJudgment(this.tokens);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferredType = inferrer.thisType ?? const DynamicType();
    inferrer.listener.thisExpression(this, fileOffset, tokens, inferredType);
    return null;
  }
}

class ThrowJudgment extends Throw implements ExpressionJudgment {
  final ThrowTokens tokens;
  final kernel.Expression desugaredError;

  DartType inferredType;

  ExpressionJudgment get judgment => expression;

  ThrowJudgment(this.tokens, Expression expression, {this.desugaredError})
      : super(expression);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferrer.inferExpression(judgment, const UnknownType(), false);
    inferredType = const BottomType();
    inferrer.listener.throw_(this, fileOffset, tokens, null, inferredType);
    if (desugaredError != null) {
      parent.replaceChild(this, desugaredError);
      parent = null;
    }
    return null;
  }
}

/// Synthetic judgment class representing a statement that is not allowed at
/// the location it was found, and should be replaced with an error.
class InvalidStatementJudgment extends ExpressionStatement
    implements StatementJudgment {
  final kernel.Expression desugaredError;
  final StatementJudgment statement;

  InvalidStatementJudgment(this.desugaredError, this.statement)
      : super(new NullLiteral());

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    inferrer.inferStatement(statement);

    // If this judgment is a part of a Block, replace it there.
    // Otherwise, the parent would be a FunctionNode, but not yet.
    if (parent is Block) {
      parent.replaceChild(this, new ExpressionStatement(desugaredError));
      parent = null;
    }
  }
}

/// Concrete shadow object representing a catch clause.
class CatchJudgment extends Catch {
  CatchStatementTokens tokens;
  CatchJudgment(this.tokens, VariableDeclaration exception, Statement body,
      {DartType guard: const DynamicType(), VariableDeclaration stackTrace})
      : super(exception, body, guard: guard, stackTrace: stackTrace);

  VariableDeclarationJudgment get exceptionJudgment => exception;

  VariableDeclarationJudgment get stackTraceJudgment => stackTrace;

  StatementJudgment get bodyJudgment => body;

  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    inferrer.inferStatement(bodyJudgment);
    inferrer.listener.catchStatement(
        this,
        fileOffset,
        tokens,
        null,
        null, // body
        exceptionJudgment?.createBinder(inferrer),
        exceptionJudgment?.type,
        stackTraceJudgment?.createBinder(inferrer),
        stackTraceJudgment?.type);
  }
}

/// Concrete shadow object representing a try-catch block in kernel form.
class TryCatchJudgment extends TryCatch implements StatementJudgment {
  TryCatchJudgment(Statement body, List<Catch> catches) : super(body, catches);

  StatementJudgment get bodyJudgment => body;

  List<CatchJudgment> get catchJudgments => catches.cast();

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    inferrer.inferStatement(bodyJudgment);
    for (var catch_ in catchJudgments) {
      catch_.infer(inferrer);
    }
    inferrer.listener.tryCatch(this, fileOffset);
  }
}

/// Concrete shadow object representing a try-finally block in kernel form.
class TryFinallyJudgment extends TryFinally implements StatementJudgment {
  TryFinallyTokens tokens;
  final List<Catch> catches;

  TryFinallyJudgment(
      this.tokens, Statement body, this.catches, Statement finalizer)
      : super(body, finalizer);

  List<CatchJudgment> get catchJudgments => catches?.cast();

  StatementJudgment get finalizerJudgment => finalizer;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    inferrer.inferStatement(body);
    if (catchJudgments != null) {
      for (var catch_ in catchJudgments) {
        catch_.infer(inferrer);
      }
      body = new TryCatch(body, catches)..parent = this;
    }
    inferrer.inferStatement(finalizerJudgment);
    inferrer.listener
        .tryFinally(this, fileOffset, tokens, body, catches, finalizer);
  }
}

/// Concrete implementation of [TypeInferenceEngine] specialized to work with
/// kernel objects.
class ShadowTypeInferenceEngine extends TypeInferenceEngine {
  ShadowTypeInferenceEngine(Instrumentation instrumentation, bool strongMode)
      : super(instrumentation, strongMode);

  @override
  TypeInferrer createDisabledTypeInferrer() =>
      new TypeInferrerDisabled(typeSchemaEnvironment);

  @override
  ShadowTypeInferrer createLocalTypeInferrer(
      Uri uri,
      TypeInferenceListener<int, Node, int> listener,
      InterfaceType thisType,
      SourceLibraryBuilder library) {
    return new ShadowTypeInferrer._(
        this, uri, listener, false, thisType, library);
  }

  @override
  ShadowTypeInferrer createTopLevelTypeInferrer(
      TypeInferenceListener<int, Node, int> listener,
      InterfaceType thisType,
      ShadowField field) {
    return field._typeInferrer = new ShadowTypeInferrer._(
        this, field.fileUri, listener, true, thisType, null);
  }

  @override
  ShadowTypeInferrer getFieldTypeInferrer(ShadowField field) {
    return field._typeInferrer;
  }
}

/// Concrete implementation of [TypeInferrer] specialized to work with kernel
/// objects.
class ShadowTypeInferrer extends TypeInferrerImpl {
  @override
  final typePromoter;

  ShadowTypeInferrer._(
      ShadowTypeInferenceEngine engine,
      Uri uri,
      TypeInferenceListener<int, Node, int> listener,
      bool topLevel,
      InterfaceType thisType,
      SourceLibraryBuilder library)
      : typePromoter = new ShadowTypePromoter(engine.typeSchemaEnvironment),
        super(engine, uri, listener, topLevel, thisType, library);

  @override
  Expression getFieldInitializer(ShadowField field) {
    return field.initializer;
  }

  @override
  DartType inferExpression<Expression, Statement, Initializer, Type>(
      kernel.Expression expression, DartType typeContext, bool typeNeeded,
      {bool isVoidAllowed: false}) {
    // `null` should never be used as the type context.  An instance of
    // `UnknownType` should be used instead.
    assert(typeContext != null);

    // It isn't safe to do type inference on an expression without a parent,
    // because type inference might cause us to have to replace one expression
    // with another, and we can only replace a node if it has a parent pointer.
    assert(expression.parent != null);

    // For full (non-top level) inference, we need access to the
    // ExpressionGeneratorHelper so that we can perform error recovery.
    assert(isTopLevel || helper != null);

    // When doing top level inference, we skip subexpressions whose type isn't
    // needed so that we don't induce bogus dependencies on fields mentioned in
    // those subexpressions.
    if (!typeNeeded && isTopLevel) return null;

    if (expression is ExpressionJudgment) {
      // Use polymorphic dispatch on [KernelExpression] to perform whatever kind
      // of type inference is correct for this kind of statement.
      // TODO(paulberry): experiment to see if dynamic dispatch would be better,
      // so that the type hierarchy will be simpler (which may speed up "is"
      // checks).
      expression.infer(this, typeContext);
      DartType inferredType = expression.inferredType;
      if (inferredType is VoidType && !isVoidAllowed) {
        if (expression.parent is! ArgumentsJudgment) {
          helper?.addProblem(
              messageVoidExpression, expression.fileOffset, noLength);
        }
      }
      return inferredType;
    } else {
      // Encountered an expression type for which type inference is not yet
      // implemented, so just infer dynamic for now.
      // TODO(paulberry): once the BodyBuilder uses shadow classes for
      // everything, this case should no longer be needed.
      return typeNeeded ? const DynamicType() : null;
    }
  }

  @override
  DartType inferFieldTopLevel<Expression, Statement, Initializer, Type>(
      ShadowField field, bool typeNeeded) {
    if (field.initializer == null) return const DynamicType();
    return inferExpression(field.initializer, const UnknownType(), typeNeeded,
        isVoidAllowed: true);
  }

  @override
  void inferInitializer<Expression, Statement, Initializer, Type>(
      InferenceHelper helper, kernel.Initializer initializer) {
    assert(initializer is InitializerJudgment);
    this.helper = helper;
    // Use polymorphic dispatch on [KernelInitializer] to perform whatever
    // kind of type inference is correct for this kind of initializer.
    // TODO(paulberry): experiment to see if dynamic dispatch would be better,
    // so that the type hierarchy will be simpler (which may speed up "is"
    // checks).
    InitializerJudgment kernelInitializer = initializer;
    kernelInitializer.infer(this);
    this.helper = null;
  }

  @override
  void inferStatement<Expression, Statement, Initializer, Type>(
      Statement statement) {
    // For full (non-top level) inference, we need access to the
    // ExpressionGeneratorHelper so that we can perform error recovery.
    if (!isTopLevel) assert(helper != null);

    if (statement is StatementJudgment) {
      // Use polymorphic dispatch on [KernelStatement] to perform whatever kind
      // of type inference is correct for this kind of statement.
      // TODO(paulberry): experiment to see if dynamic dispatch would be better,
      // so that the type hierarchy will be simpler (which may speed up "is"
      // checks).
      return statement.infer(this);
    } else {
      // Encountered a statement type for which type inference is not yet
      // implemented, so just skip it for now.
      // TODO(paulberry): once the BodyBuilder uses shadow classes for
      // everything, this case should no longer be needed.
    }
  }
}

class TypeLiteralJudgment extends TypeLiteral implements ExpressionJudgment {
  DartType inferredType;

  TypeLiteralJudgment(DartType type) : super(type);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferredType = inferrer.coreTypes.typeClass.rawType;
    inferrer.listener.typeLiteral(this, fileOffset, type, inferredType);
    return null;
  }
}

/// Concrete implementation of [TypePromoter] specialized to work with kernel
/// objects.
class ShadowTypePromoter extends TypePromoterImpl {
  ShadowTypePromoter(TypeSchemaEnvironment typeSchemaEnvironment)
      : super(typeSchemaEnvironment);

  @override
  int getVariableFunctionNestingLevel(VariableDeclaration variable) {
    if (variable is VariableDeclarationJudgment) {
      return variable._functionNestingLevel;
    } else {
      // Hack to deal with the fact that BodyBuilder still creates raw
      // VariableDeclaration objects sometimes.
      // TODO(paulberry): get rid of this once the type parameter is
      // KernelVariableDeclaration.
      return 0;
    }
  }

  @override
  bool isPromotionCandidate(VariableDeclaration variable) {
    assert(variable is VariableDeclarationJudgment);
    VariableDeclarationJudgment kernelVariableDeclaration = variable;
    return !kernelVariableDeclaration._isLocalFunction;
  }

  @override
  bool sameExpressions(Expression a, Expression b) {
    return identical(a, b);
  }

  @override
  void setVariableMutatedAnywhere(VariableDeclaration variable) {
    if (variable is VariableDeclarationJudgment) {
      variable._mutatedAnywhere = true;
    } else {
      // Hack to deal with the fact that BodyBuilder still creates raw
      // VariableDeclaration objects sometimes.
      // TODO(paulberry): get rid of this once the type parameter is
      // KernelVariableDeclaration.
    }
  }

  @override
  void setVariableMutatedInClosure(VariableDeclaration variable) {
    if (variable is VariableDeclarationJudgment) {
      variable._mutatedInClosure = true;
    } else {
      // Hack to deal with the fact that BodyBuilder still creates raw
      // VariableDeclaration objects sometimes.
      // TODO(paulberry): get rid of this once the type parameter is
      // KernelVariableDeclaration.
    }
  }

  @override
  bool wasVariableMutatedAnywhere(VariableDeclaration variable) {
    if (variable is VariableDeclarationJudgment) {
      return variable._mutatedAnywhere;
    } else {
      // Hack to deal with the fact that BodyBuilder still creates raw
      // VariableDeclaration objects sometimes.
      // TODO(paulberry): get rid of this once the type parameter is
      // KernelVariableDeclaration.
      return true;
    }
  }
}

class VariableAssignmentJudgment extends ComplexAssignmentJudgment {
  VariableAssignmentJudgment(ExpressionJudgment rhs) : super(rhs);

  @override
  DartType _getWriteType(ShadowTypeInferrer inferrer) {
    VariableSet write = this.write;
    return write.variable.type;
  }

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    DartType readType;
    var read = this.read;
    if (read is VariableGet) {
      readType = read.promotedType ?? read.variable.type;
    }
    DartType writeContext = const UnknownType();
    var write = this.write;
    if (write is VariableSet) {
      writeContext = write.variable.type;
      if (read != null) {
        _storeLetType(inferrer, read, writeContext);
      }
    }
    var inferredResult = _inferRhs(inferrer, readType, writeContext);
    inferrer.listener.variableAssign(
        this,
        write.fileOffset,
        writeContext,
        write is VariableSet
            ? (write.variable as VariableDeclarationJudgment)
                .createBinder(inferrer)
            : null,
        inferredResult.combiner,
        inferredType);
    _replaceWithDesugared();
    return null;
  }
}

/// Concrete shadow object representing a variable declaration in kernel form.
class VariableDeclarationJudgment extends VariableDeclaration
    implements StatementJudgment {
  final bool forSyntheticToken;

  final bool _implicitlyTyped;

  final int _functionNestingLevel;

  bool _mutatedInClosure = false;

  bool _mutatedAnywhere = false;

  final bool _isLocalFunction;

  Object binder;

  /// The same [annotations] list is used for all [VariableDeclarationJudgment]s
  /// of a variable declaration statement. But we need to perform inference
  /// only once. So, we set this flag to `false` for the second and subsequent
  /// judgments.
  bool infersAnnotations = true;

  VariableDeclarationJudgment(String name, this._functionNestingLevel,
      {this.forSyntheticToken: false,
      Expression initializer,
      DartType type,
      bool isFinal: false,
      bool isConst: false,
      bool isFieldFormal: false,
      bool isCovariant: false,
      bool isLocalFunction: false})
      : _implicitlyTyped = type == null,
        _isLocalFunction = isLocalFunction,
        super(name,
            initializer: initializer,
            type: type ?? const DynamicType(),
            isFinal: isFinal,
            isConst: isConst,
            isFieldFormal: isFieldFormal,
            isCovariant: isCovariant);

  VariableDeclarationJudgment.forEffect(
      Expression initializer, this._functionNestingLevel)
      : forSyntheticToken = false,
        _implicitlyTyped = false,
        _isLocalFunction = false,
        super.forValue(initializer);

  VariableDeclarationJudgment.forValue(
      Expression initializer, this._functionNestingLevel)
      : forSyntheticToken = false,
        _implicitlyTyped = true,
        _isLocalFunction = false,
        super.forValue(initializer);

  List<Expression> get annotationJudgments => annotations;

  ExpressionJudgment get initializerJudgment => initializer;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    if (annotationJudgments.isNotEmpty) {
      if (infersAnnotations) {
        inferrer.inferMetadataKeepingHelper(annotationJudgments);
      }

      // After the inference was done on the annotations, we may clone them for
      // this instance of VariableDeclaration in order to avoid having the same
      // annotation node for two VariableDeclaration nodes in a situation like
      // the following:
      //
      //     class Foo { const Foo(List<String> list); }
      //
      //     @Foo(const [])
      //     var x, y;
      CloneVisitor cloner = new CloneVisitor();
      for (int i = 0; i < annotations.length; ++i) {
        kernel.Expression annotation = annotations[i];
        if (annotation.parent != this) {
          annotations[i] = cloner.clone(annotation);
          annotations[i].parent = this;
        }
      }
    }

    var initializerJudgment = this.initializerJudgment;
    var declaredType = _implicitlyTyped ? const UnknownType() : type;
    DartType inferredType;
    DartType initializerType;
    if (initializerJudgment != null) {
      inferrer.inferExpression(initializerJudgment, declaredType,
          !inferrer.isTopLevel || _implicitlyTyped,
          isVoidAllowed: true);
      initializerType = initializerJudgment.inferredType;
      inferredType = inferrer.inferDeclarationType(initializerType);
    } else {
      inferredType = const DynamicType();
    }
    if (inferrer.strongMode && _implicitlyTyped) {
      inferrer.instrumentation?.record(inferrer.uri, fileOffset, 'type',
          new InstrumentationValueForType(inferredType));
      type = inferredType;
    }
    if (initializer != null) {
      var replacedInitializer = inferrer.ensureAssignable(
          type, initializerType, initializer, fileOffset,
          isVoidAllowed: type is VoidType);
      if (replacedInitializer != null) {
        initializer = replacedInitializer;
      }
    }
    inferrer.listener.variableDeclaration(
        createBinder(inferrer), _implicitlyTyped ? inferredType : type);
  }

  Object createBinder(ShadowTypeInferrer inferrer) =>
      binder ??= _isLocalFunction
          ? inferrer.listener
              .binderForFunctionDeclaration(this, fileOffset, name)
          : inferrer.listener.binderForVariableDeclaration(
              this, fileOffset, name, forSyntheticToken);

  /// Determine whether the given [VariableDeclarationJudgment] had an implicit
  /// type.
  ///
  /// This is static to avoid introducing a method that would be visible to
  /// the kernel.
  static bool isImplicitlyTyped(VariableDeclarationJudgment variable) =>
      variable._implicitlyTyped;

  /// Determines whether the given [VariableDeclarationJudgment] represents a
  /// local function.
  ///
  /// This is static to avoid introducing a method that would be visible to the
  /// kernel.
  static bool isLocalFunction(VariableDeclarationJudgment variable) =>
      variable._isLocalFunction;
}

/// Synthetic judgment class representing an attempt to invoke an unresolved
/// target.
class UnresolvedTargetInvocationJudgment extends SyntheticExpressionJudgment {
  final ArgumentsJudgment argumentsJudgment;

  UnresolvedTargetInvocationJudgment(
      kernel.Expression desugared, this.argumentsJudgment)
      : super(desugared);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    var result = super.infer(inferrer, typeContext);
    inferrer.inferInvocation(
        typeContext,
        fileOffset,
        TypeInferrerImpl.unknownFunction,
        const DynamicType(),
        argumentsJudgment);
    inferrer.listener.staticInvocation(this, fileOffset, null,
        argumentsJudgment.types, null, null, inferredType);
    return result;
  }
}

/// Synthetic judgment class representing an attempt to assign to an unresolved
/// variable.
class UnresolvedVariableAssignmentJudgment extends SyntheticExpressionJudgment {
  final bool isCompound;
  final ExpressionJudgment rhs;

  UnresolvedVariableAssignmentJudgment(
      kernel.Expression desugared, this.isCompound, this.rhs)
      : super(desugared);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferrer.inferExpression(rhs, const UnknownType(), true);
    inferredType = isCompound ? const DynamicType() : rhs.inferredType;
    inferrer.listener.variableAssign(
        this, fileOffset, const DynamicType(), null, null, inferredType);
    return super.infer(inferrer, typeContext);
  }
}

/// Synthetic judgment class representing an attempt to apply a prefix or
/// postfix operator to an unresolved variable.
class UnresolvedVariableUnaryJudgment extends SyntheticExpressionJudgment {
  final int offset;
  final bool isSynthetic;

  UnresolvedVariableUnaryJudgment(
      kernel.Expression desugared, this.offset, this.isSynthetic)
      : super(desugared);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferrer.listener.variableGet(
        this, offset, isSynthetic, false, null, const DynamicType());
    inferrer.listener.variableAssign(
        this, fileOffset, const DynamicType(), null, null, inferredType);
    return super.infer(inferrer, typeContext);
  }
}

/// Synthetic judgment class representing an attempt to read an unresolved
/// variable.
class UnresolvedVariableGetJudgment extends SyntheticExpressionJudgment {
  final bool forSyntheticToken;

  UnresolvedVariableGetJudgment(
      kernel.Expression desugared, this.forSyntheticToken)
      : super(desugared);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferrer.listener.variableGet(
        this, fileOffset, forSyntheticToken, false, null, const DynamicType());
    return super.infer(inferrer, typeContext);
  }
}

/// Concrete shadow object representing a read from a variable in kernel form.
class VariableGetJudgment extends VariableGet implements ExpressionJudgment {
  DartType inferredType;

  final TypePromotionFact _fact;

  final TypePromotionScope _scope;

  VariableGetJudgment(VariableDeclaration variable, this._fact, this._scope)
      : super(variable);

  /// Return `true` if the given [variable] declaration occurs in a let
  /// expression that is, or is part of, a cascade expression.
  bool _isInCascade() {
    TreeNode ancestor = variable.parent;
    while (ancestor is Let) {
      if (ancestor is CascadeJudgment) {
        return true;
      }
      ancestor = ancestor.parent;
    }
    return false;
  }

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    VariableDeclarationJudgment variable = this.variable;
    bool mutatedInClosure = variable._mutatedInClosure;
    DartType declaredOrInferredType = variable.type;

    DartType promotedType = inferrer.typePromoter
        .computePromotedType(_fact, _scope, mutatedInClosure);
    if (promotedType != null) {
      inferrer.instrumentation?.record(inferrer.uri, fileOffset, 'promotedType',
          new InstrumentationValueForType(promotedType));
    }
    this.promotedType = promotedType;
    var type = promotedType ?? declaredOrInferredType;
    if (variable._isLocalFunction) {
      type = inferrer.instantiateTearOff(type, typeContext, this);
    }
    inferredType = type;
    inferrer.listener.variableGet(this, fileOffset, false, _isInCascade(),
        variable.createBinder(inferrer), inferredType);
    return null;
  }
}

/// Concrete shadow object representing a while loop in kernel form.
class WhileJudgment extends WhileStatement implements StatementJudgment {
  WhileStatementTokens tokens;

  WhileJudgment(this.tokens, Expression condition, Statement body)
      : super(condition, body);

  ExpressionJudgment get conditionJudgment => condition;

  StatementJudgment get bodyJudgment => body;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    var conditionJudgment = this.conditionJudgment;
    var expectedType = inferrer.coreTypes.boolClass.rawType;
    inferrer.inferExpression(
        conditionJudgment, expectedType, !inferrer.isTopLevel);
    inferrer.ensureAssignable(expectedType, conditionJudgment.inferredType,
        condition, condition.fileOffset);
    inferrer.inferStatement(bodyJudgment);
    inferrer.listener.whileStatement(this, fileOffset, tokens, null, null);
  }
}

/// Concrete shadow object representing a yield statement in kernel form.
class YieldJudgment extends YieldStatement implements StatementJudgment {
  YieldStatementTokens tokens;

  YieldJudgment(this.tokens, bool isYieldStar, Expression expression)
      : super(expression, isYieldStar: isYieldStar);

  ExpressionJudgment get judgment => expression;

  @override
  void infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer) {
    var judgment = this.judgment;
    var closureContext = inferrer.closureContext;
    if (closureContext.isGenerator) {
      var typeContext = closureContext.returnOrYieldContext;
      if (isYieldStar && typeContext != null) {
        typeContext = inferrer.wrapType(
            typeContext,
            closureContext.isAsync
                ? inferrer.coreTypes.streamClass
                : inferrer.coreTypes.iterableClass);
      }
      inferrer.inferExpression(judgment, typeContext, true);
    } else {
      inferrer.inferExpression(judgment, const UnknownType(), true);
    }
    closureContext.handleYield(
        inferrer, isYieldStar, judgment.inferredType, expression, fileOffset);
    inferrer.listener.yieldStatement(this, fileOffset, tokens, null);
  }
}

/// Concrete shadow object representing a deferred load library call.
class LoadLibraryJudgment extends LoadLibrary implements ExpressionJudgment {
  final Arguments arguments;

  DartType inferredType;

  LoadLibraryJudgment(LibraryDependency import, this.arguments) : super(import);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferredType =
        inferrer.typeSchemaEnvironment.futureType(const DynamicType());
    if (arguments != null) {
      var calleeType = new FunctionType([], inferredType);
      inferrer.inferInvocation(typeContext, fileOffset, calleeType,
          calleeType.returnType, argumentJudgments);
      inferrer.listener.loadLibrary(this, arguments.fileOffset,
          import.targetLibrary, calleeType, inferredType);
    }
    return null;
  }
}

/// Concrete shadow object representing a tear-off of a `loadLibrary` function.
class LoadLibraryTearOffJudgment extends StaticGet
    implements ExpressionJudgment {
  final LibraryDependency import;

  DartType inferredType;

  LoadLibraryTearOffJudgment(this.import, Procedure target) : super(target);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferredType = new FunctionType(
        [], inferrer.typeSchemaEnvironment.futureType(const DynamicType()));
    inferrer.listener.loadLibraryTearOff(
        this, fileOffset, import.targetLibrary, inferredType);
    return null;
  }
}

/// Concrete shadow object representing a deferred library-is-loaded check.
class CheckLibraryIsLoadedJudgment extends CheckLibraryIsLoaded
    implements ExpressionJudgment {
  DartType inferredType;

  CheckLibraryIsLoadedJudgment(LibraryDependency import) : super(import);

  @override
  Expression infer<Expression, Statement, Initializer, Type>(
      ShadowTypeInferrer inferrer, DartType typeContext) {
    inferredType = inferrer.typeSchemaEnvironment.objectType;
    return null;
  }
}

/// Concrete shadow object representing a named expression.
class NamedExpressionJudgment extends NamedExpression {
  NamedExpressionTokens tokens;

  NamedExpressionJudgment(this.tokens, String nameLexeme, Expression value)
      : super(nameLexeme, value);

  ExpressionJudgment get judgment => value;
}

/// The result of inference for a RHS of an assignment.
class _ComplexAssignmentInferenceResult {
  /// The resolved combiner [Procedure], e.g. `operator+` for `a += 2`, or
  /// `null` if the assignment is not compound.
  final Procedure combiner;

  _ComplexAssignmentInferenceResult(this.combiner);
}

class _UnfinishedCascade extends Expression {
  accept(v) => unsupported("accept", -1, null);

  accept1(v, arg) => unsupported("accept1", -1, null);

  getStaticType(types) => unsupported("getStaticType", -1, null);

  transformChildren(v) => unsupported("transformChildren", -1, null);

  visitChildren(v) => unsupported("visitChildren", -1, null);
}