pkg/dev_compiler/lib/src/kernel/compiler.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.

import 'dart:collection';
import 'dart:math' show max, min;

import 'package:front_end/src/fasta/type_inference/type_schema_environment.dart';
import 'package:kernel/class_hierarchy.dart';
import 'package:kernel/core_types.dart';
import 'package:kernel/kernel.dart' hide ConstantVisitor;
import 'package:kernel/type_algebra.dart';
import 'package:kernel/type_environment.dart';
import 'package:source_span/source_span.dart' show SourceLocation;

import '../compiler/js_names.dart' as JS;
import '../compiler/js_utils.dart' as JS;
import '../compiler/module_builder.dart' show pathToJSIdentifier;
import '../js_ast/js_ast.dart' as JS;
import '../js_ast/js_ast.dart' show js;
import '../js_ast/source_map_printer.dart' show NodeEnd, NodeSpan, HoverComment;
import 'js_interop.dart';
import 'js_typerep.dart';
import 'kernel_helpers.dart';
import 'native_types.dart';
import 'nullable_inference.dart';
import 'property_model.dart';
import 'type_table.dart';

class ProgramCompiler
    implements
        StatementVisitor<JS.Statement>,
        ExpressionVisitor<JS.Expression>,
        DartTypeVisitor<JS.Expression> {
  /// The list of output module items, in the order they need to be emitted in.
  final _moduleItems = <JS.ModuleItem>[];

  /// The set of libraries we are currently compiling, and the temporaries used
  /// to refer to them.
  ///
  /// We sometimes special case codegen for a single library, as it simplifies
  /// name scoping requirements.
  final _libraries = new Map<Library, JS.Identifier>.identity();

  /// Maps a library URI import, that is not in [_libraries], to the
  /// corresponding Kernel summary module we imported it with.
  final _importToSummary = new Map<Library, Component>.identity();

  /// Maps a summary to the file URI we used to load it from disk.
  final _summaryToUri = new Map<Component, Uri>.identity();

  /// Imported libraries, and the temporaries used to refer to them.
  final _imports = new Map<Library, JS.TemporaryId>();

  /// The variable for the current catch clause
  VariableDeclaration _catchParameter;

  /// In an async* function, this represents the stream controller parameter.
  JS.TemporaryId _asyncStarController;

  // TODO(jmesserly): fuse this with notNull check.
  final _privateNames = new HashMap<Library, HashMap<String, JS.TemporaryId>>();

  JS.Identifier _extensionSymbolsModule;
  final _extensionSymbols = new Map<String, JS.TemporaryId>();

  JS.Identifier _runtimeModule;
  final namedArgumentTemp = new JS.TemporaryId('opts');

  Set<Class> _pendingClasses;

  /// Temporary variables mapped to their corresponding JavaScript variable.
  final _tempVariables = <VariableDeclaration, JS.TemporaryId>{};

  /// Let variables collected for the given function.
  List<JS.TemporaryId> _letVariables;

  /// The class when it's emitting top-level code, used to order classes when
  /// they extend each other.
  ///
  /// This is not used when inside method bodies, or for other type information
  /// such as `implements`.
  Class _classEmittingTopLevel;

  /// The current element being loaded.
  /// We can use this to determine if we're loading top-level code or not:
  ///
  ///     _currentClass == _classEmittingTopLevel
  ///
  Class _currentClass;

  /// The current source file URI for emitting in the source map.
  Uri _currentUri;

  Component _component;

  Library _currentLibrary;

  FunctionNode _currentFunction;

  List<TypeParameter> _typeParamInConst;

  /// Whether we are currently generating code for the body of a `JS()` call.
  bool _isInForeignJS = false;

  /// Table of named and possibly hoisted types.
  TypeTable _typeTable;

  /// The global extension type table.
  // TODO(jmesserly): rename to `_nativeTypes`
  final NativeTypeSet _extensionTypes;

  final CoreTypes coreTypes;

  final TypeEnvironment types;

  /// Information about virtual and overridden fields/getters/setters in the
  /// class we're currently compiling, or `null` if we aren't compiling a class.
  ClassPropertyModel _classProperties;

  /// Information about virtual fields for all libraries in the current build
  /// unit.
  final virtualFields = new VirtualFieldModel();

  final JSTypeRep _typeRep;

  bool _superAllowed = true;

  final _superHelpers = new Map<String, JS.Method>();

  final bool emitMetadata;
  final bool replCompile;

  final Map<String, String> declaredVariables;

  // Compilation of Kernel's [BreakStatement].
  //
  // Kernel represents Dart's `break` and `continue` uniformly as
  // [BreakStatement], by representing a loop continue as a break from the
  // loop's body.  [BreakStatement] always targets an enclosing
  // [LabeledStatement] statement directly without naming it.  (Continue to
  // a labeled switch case is not represented by a [BreakStatement].)
  //
  // We prefer to compile to `continue` where possible and to avoid labeling
  // statements where it is not necessary.  We maintain some state to track
  // which statements can be targets of break or continue without a label, which
  // statements must be labeled to be targets, and the labels that have been
  // assigned.

  /// A list of statements that can be the target of break without a label.
  ///
  /// A [BreakStatement] targeting any [LabeledStatement] in this list can be
  /// compiled to a break without a label.  All the statements in the list have
  /// the same effective target which must compile to something that can be
  /// targeted by break in JS.  This list and [_currentContinueTargets] are
  /// disjoint.
  List<LabeledStatement> _currentBreakTargets = [];

  /// A list of statements that can be the target of a continue without a label.
  ///
  /// A [BreakStatement] targeting any [LabeledStatement] in this list can be
  /// compiled to a continue without a label.  All the statements in this list
  /// have the same effective target which must compile to something that can be
  /// targeted by continue in JS.  This list and [_currentBreakTargets] are
  /// disjoint.
  List<LabeledStatement> _currentContinueTargets = [];

  /// A map from labeled statements to their 'effective targets'.
  ///
  /// The effective target of a labeled loop body is the enclosing loop.  A
  /// [BreakStatement] targeting this statement can be compiled to `continue`
  /// either with or without a label.  The effective target of a labeled
  /// statement that is not a loop body is the outermost non-labeled statement
  /// that it encloses.  A [BreakStatement] targeting this statement can be
  /// compiled to `break` either with or without a label.
  final _effectiveTargets = new HashMap<LabeledStatement, Statement>.identity();

  /// A map from effective targets to their label names.
  ///
  /// If the target needs to be labeled when compiled to JS, because it was
  /// targeted by a break or continue with a label, then this map contains the
  /// label name that was assigned to it.
  final _labelNames = new HashMap<Statement, String>.identity();

  final Class _jsArrayClass;
  final Class _jsBoolClass;
  final Class _jsNumberClass;
  final Class _jsStringClass;
  final Class privateSymbolClass;
  final Class linkedHashMapImplClass;
  final Class identityHashMapImplClass;
  final Class linkedHashSetImplClass;
  final Class identityHashSetImplClass;
  final Class syncIterableClass;

  /// The dart:async `StreamIterator<T>` type.
  final Class _asyncStreamIteratorClass;

  final ConstantVisitor _constants;

  final NullableInference _nullableInference;

  factory ProgramCompiler(Component component,
      {bool emitMetadata: true,
      bool replCompile: false,
      Map<String, String> declaredVariables: const {}}) {
    var nativeTypes = new NativeTypeSet(component);
    var types = new TypeSchemaEnvironment(
        nativeTypes.coreTypes, new ClassHierarchy(component), true);
    return new ProgramCompiler._(
        nativeTypes, new JSTypeRep(types, nativeTypes.sdk),
        emitMetadata: emitMetadata,
        replCompile: replCompile,
        declaredVariables: declaredVariables);
  }

  ProgramCompiler._(NativeTypeSet nativeTypes, this._typeRep,
      {this.emitMetadata, this.replCompile, this.declaredVariables})
      : _extensionTypes = nativeTypes,
        types = _typeRep.types,
        coreTypes = nativeTypes.coreTypes,
        _constants = new ConstantVisitor(nativeTypes.coreTypes),
        _jsArrayClass =
            nativeTypes.sdk.getClass('dart:_interceptors', 'JSArray'),
        _jsBoolClass = nativeTypes.sdk.getClass('dart:_interceptors', 'JSBool'),
        _jsNumberClass =
            nativeTypes.sdk.getClass('dart:_interceptors', 'JSNumber'),
        _jsStringClass =
            nativeTypes.sdk.getClass('dart:_interceptors', 'JSString'),
        _asyncStreamIteratorClass =
            nativeTypes.sdk.getClass('dart:async', 'StreamIterator'),
        privateSymbolClass =
            nativeTypes.sdk.getClass('dart:_js_helper', 'PrivateSymbol'),
        linkedHashMapImplClass =
            nativeTypes.sdk.getClass('dart:_js_helper', 'LinkedMap'),
        identityHashMapImplClass =
            nativeTypes.sdk.getClass('dart:_js_helper', 'IdentityMap'),
        linkedHashSetImplClass =
            nativeTypes.sdk.getClass('dart:collection', '_HashSet'),
        identityHashSetImplClass =
            nativeTypes.sdk.getClass('dart:collection', '_IdentityHashSet'),
        syncIterableClass =
            nativeTypes.sdk.getClass('dart:_js_helper', 'SyncIterable'),
        _nullableInference = new NullableInference(_typeRep);

  ClassHierarchy get hierarchy => types.hierarchy;

  JS.Program emitProgram(
      Component p, List<Component> summaries, List<Uri> summaryUris) {
    if (_moduleItems.isNotEmpty) {
      throw new StateError('Can only call emitModule once.');
    }
    _component = p;

    for (var i = 0; i < summaries.length; i++) {
      var summary = summaries[i];
      var summaryUri = summaryUris[i];
      for (var l in summary.libraries) {
        assert(!_importToSummary.containsKey(l));
        _importToSummary[l] = summary;
        _summaryToUri[summary] = summaryUri;
      }
    }

    var libraries = p.libraries.where((l) => !l.isExternal);
    var ddcRuntime =
        libraries.firstWhere(isSdkInternalRuntime, orElse: () => null);
    if (ddcRuntime != null) {
      // Don't allow these to be renamed when we're building the SDK.
      // There is JS code in dart:* that depends on their names.
      _runtimeModule = new JS.Identifier('dart');
      _extensionSymbolsModule = new JS.Identifier('dartx');
      _nullableInference.allowNotNullDeclarations = true;
    } else {
      // Otherwise allow these to be renamed so users can write them.
      _runtimeModule = new JS.TemporaryId('dart');
      _extensionSymbolsModule = new JS.TemporaryId('dartx');
    }
    _typeTable = new TypeTable(_runtimeModule);

    // Initialize our library variables.
    var items = <JS.ModuleItem>[];
    var exports = <JS.NameSpecifier>[];
    var root = new JS.Identifier('_root');
    items.add(js.statement('const # = Object.create(null)', [root]));

    void emitLibrary(JS.Identifier id) {
      items.add(js.statement('const # = Object.create(#)', [id, root]));
      exports.add(new JS.NameSpecifier(id));
    }

    for (var library in libraries) {
      var libraryTemp = library == ddcRuntime
          ? _runtimeModule
          : new JS.TemporaryId(jsLibraryName(library));
      _libraries[library] = libraryTemp;
      emitLibrary(libraryTemp);
    }

    // dart:_runtime has a magic module that holds extension method symbols.
    // TODO(jmesserly): find a cleaner design for this.
    if (ddcRuntime != null) emitLibrary(_extensionSymbolsModule);

    items.add(new JS.ExportDeclaration(new JS.ExportClause(exports)));

    // Collect all class/type Element -> Node mappings
    // in case we need to forward declare any classes.
    _pendingClasses = new HashSet.identity();
    for (var l in libraries) {
      _pendingClasses.addAll(l.classes);
    }

    // Add implicit dart:core dependency so it is first.
    emitLibraryName(coreTypes.coreLibrary);

    // Visit each library and emit its code.
    //
    // NOTE: clases are not necessarily emitted in this order.
    // Order will be changed as needed so the resulting code can execute.
    // This is done by forward declaring items.
    libraries.forEach(_emitLibrary);

    // Visit directives (for exports)
    libraries.forEach(_emitExports);

    // Declare imports
    _finishImports(items);
    // Initialize extension symbols
    _extensionSymbols.forEach((name, id) {
      JS.Expression value =
          new JS.PropertyAccess(_extensionSymbolsModule, _propertyName(name));
      if (ddcRuntime != null) {
        value = js.call('# = Symbol(#)', [value, js.string("dartx.$name")]);
      }
      items.add(js.statement('const # = #;', [id, value]));
    });

    // Discharge the type table cache variables and
    // hoisted definitions.
    items.addAll(_typeTable.discharge());

    // Add the module's code (produced by visiting compilation units, above)
    _copyAndFlattenBlocks(items, _moduleItems);

    // Build the module.
    return new JS.Program(items, name: p.root.name);
  }

  /// Flattens blocks in [items] to a single list.
  ///
  /// This will not flatten blocks that are marked as being scopes.
  void _copyAndFlattenBlocks(
      List<JS.ModuleItem> result, Iterable<JS.ModuleItem> items) {
    for (var item in items) {
      if (item is JS.Block && !item.isScope) {
        _copyAndFlattenBlocks(result, item.statements);
      } else if (item != null) {
        result.add(item);
      }
    }
  }

  /// Returns the canonical name to refer to the Dart library.
  JS.Identifier emitLibraryName(Library library) {
    // It's either one of the libraries in this module, or it's an import.
    return _libraries[library] ??
        _imports.putIfAbsent(
            library, () => new JS.TemporaryId(jsLibraryName(library)));
  }

  String _libraryToModule(Library library) {
    assert(!_libraries.containsKey(library));
    if (library.importUri.scheme == 'dart') {
      // TODO(jmesserly): we need to split out HTML.
      return JS.dartSdkModule;
    }
    var summary = _importToSummary[library];
    assert(summary != null);
    // TODO(jmesserly): look up the appropriate relative import path if the user
    // specified that on the command line.
    var uri = _summaryToUri[summary];
    var summaryPath = uri.path;
    var extensionIndex = summaryPath.lastIndexOf('.');
    // Note: These URIs do not contain absolute paths from the physical file
    // system, but only the relevant path within a user's project. This path
    // will match the path where the .js file is generated, so we use it as
    // the module name.
    var moduleName = summaryPath.substring(1, extensionIndex);
    return moduleName;
  }

  void _finishImports(List<JS.ModuleItem> items) {
    var modules = new Map<String, List<Library>>();

    for (var import in _imports.keys) {
      modules.putIfAbsent(_libraryToModule(import), () => []).add(import);
    }

    String coreModuleName;
    if (!_libraries.containsKey(coreTypes.coreLibrary)) {
      coreModuleName = _libraryToModule(coreTypes.coreLibrary);
    }
    modules.forEach((module, libraries) {
      // Generate import directives.
      //
      // Our import variables are temps and can get renamed. Since our renaming
      // is integrated into js_ast, it is aware of this possibility and will
      // generate an "as" if needed. For example:
      //
      //     import {foo} from 'foo';         // if no rename needed
      //     import {foo as foo$} from 'foo'; // if rename was needed
      //
      var imports =
          libraries.map((l) => new JS.NameSpecifier(_imports[l])).toList();
      if (module == coreModuleName) {
        imports.add(new JS.NameSpecifier(_runtimeModule));
        imports.add(new JS.NameSpecifier(_extensionSymbolsModule));
      }

      items.add(new JS.ImportDeclaration(
          namedImports: imports, from: js.string(module, "'")));
    });
  }

  void _emitLibrary(Library library) {
    // NOTE: this method isn't the right place to initialize per-library state.
    // Classes can be visited out of order, so this is only to catch things that
    // haven't been emitted yet.
    //
    // See _emitClass.
    assert(_currentLibrary == null);
    _currentLibrary = library;

    // `dart:_runtime` uses a different order for bootstrapping.
    bool bootstrap = isSdkInternalRuntime(library);
    if (bootstrap) _emitLibraryProcedures(library);

    library.classes.forEach(_emitClass);
    library.typedefs.forEach(_emitTypedef);
    if (bootstrap) {
      _moduleItems.add(_emitInternalSdkFields(library.fields));
    } else {
      _emitLibraryProcedures(library);
      var fields = library.fields;
      if (fields.isNotEmpty)
        _moduleItems.add(_emitLazyFields(emitLibraryName(library), fields));
    }

    _currentLibrary = null;
  }

  void _emitExports(Library library) {
    assert(_currentLibrary == null);
    _currentLibrary = library;

    library.additionalExports.forEach(_emitExport);

    _currentLibrary = null;
  }

  void _emitExport(Reference export) {
    var library = _currentLibrary;

    // We only need to export main as it is the only method part of the
    // publicly exposed JS API for a library.
    // TODO(jacobr): add a library level annotation indicating that all
    // contents of a library need to be exposed to JS.
    // https://github.com/dart-lang/sdk/issues/26368

    var node = export.node;
    if (node is Procedure && node.name.name == 'main') {
      // Don't allow redefining names from this library.
      var name = _emitTopLevelName(export.node);
      _moduleItems.add(js.statement(
          '#.# = #;', [emitLibraryName(library), name.selector, name]));
    }
  }

  /// Called to emit class declarations.
  ///
  /// During the course of emitting one item, we may emit another. For example
  ///
  ///     class D extends B { C m() { ... } }
  ///
  /// Because D depends on B, we'll emit B first if needed. However C is not
  /// used by top-level JavaScript code, so we can ignore that dependency.
  void _emitClass(Class c) {
    if (!_pendingClasses.remove(c)) return;

    var savedClass = _currentClass;
    var savedLibrary = _currentLibrary;
    var savedUri = _currentUri;
    _currentClass = c;
    types.thisType = c.thisType;
    _currentLibrary = c.enclosingLibrary;
    _currentUri = c.fileUri;

    _moduleItems.add(_emitClassDeclaration(c));

    _currentClass = savedClass;
    types.thisType = savedClass?.thisType;
    _currentLibrary = savedLibrary;
    _currentUri = savedUri;
  }

  /// To emit top-level classes, we sometimes need to reorder them.
  ///
  /// This function takes care of that, and also detects cases where reordering
  /// failed, and we need to resort to lazy loading, by marking the element as
  /// lazy. All elements need to be aware of this possibility and generate code
  /// accordingly.
  ///
  /// If we are not emitting top-level code, this does nothing, because all
  /// declarations are assumed to be available before we start execution.
  /// See [startTopLevel].
  void _declareBeforeUse(Class c) {
    if (c == null) return;
    if (identical(_currentClass, _classEmittingTopLevel)) _emitClass(c);
  }

  JS.Statement _emitClassDeclaration(Class c) {
    // If this class is annotated with `@JS`, then there is nothing to emit.
    if (findAnnotation(c, isPublicJSAnnotation) != null) return null;

    // If this is a JavaScript type, emit it now and then exit.
    var jsTypeDef = _emitJSType(c);
    if (jsTypeDef != null) return jsTypeDef;

    JS.Expression className;
    if (c.typeParameters.isNotEmpty) {
      // Generic classes will be defined inside a function that closes over the
      // type parameter. So we can use their local variable name directly.
      className = new JS.Identifier(getLocalClassName(c));
    } else {
      className = _emitTopLevelName(c);
    }

    var savedClassProperties = _classProperties;
    _classProperties =
        new ClassPropertyModel.build(types, _extensionTypes, virtualFields, c);

    var jsCtors = _defineConstructors(c, className);
    var jsMethods = _emitClassMethods(c);

    var body = <JS.Statement>[];
    _emitSuperHelperSymbols(body);
    var deferredSupertypes = <JS.Statement>[];

    // Emit the class, e.g. `core.Object = class Object { ... }`
    _defineClass(c, className, jsMethods, body, deferredSupertypes);
    body.addAll(jsCtors);

    // Emit things that come after the ES6 `class ... { ... }`.
    var jsPeerNames = _getJSPeerNames(c);
    if (jsPeerNames.length == 1 && c.typeParameters.isNotEmpty) {
      // Special handling for JSArray<E>
      body.add(_callHelperStatement('setExtensionBaseClass(#, #.global.#);',
          [className, _runtimeModule, jsPeerNames[0]]));
    }

    var finishGenericTypeTest = _emitClassTypeTests(c, className, body);

    _emitVirtualFieldSymbols(c, body);
    _emitClassSignature(c, className, body);
    _initExtensionSymbols(c);
    _defineExtensionMembers(className, body);
    _emitClassMetadata(c.annotations, className, body);

    var classDef = JS.Statement.from(body);
    var typeFormals = c.typeParameters;
    if (typeFormals.isNotEmpty) {
      classDef = _defineClassTypeArguments(
          c, typeFormals, classDef, className, deferredSupertypes);
    } else {
      body.addAll(deferredSupertypes);
    }

    body = [classDef];
    _emitStaticFields(c, body);
    if (finishGenericTypeTest != null) body.add(finishGenericTypeTest);
    for (var peer in jsPeerNames) {
      _registerExtensionType(c, peer, body);
    }

    _classProperties = savedClassProperties;
    return JS.Statement.from(body);
  }

  /// Wraps a possibly generic class in its type arguments.
  JS.Statement _defineClassTypeArguments(
      NamedNode c, List<TypeParameter> formals, JS.Statement body,
      [JS.Expression className, List<JS.Statement> deferredBaseClass]) {
    assert(formals.isNotEmpty);
    var name = getTopLevelName(c);
    var jsFormals = _emitTypeFormals(formals);
    var typeConstructor = js.call('(#) => { #; #; return #; }', [
      jsFormals,
      _typeTable.discharge(formals),
      body,
      className ?? new JS.Identifier(name)
    ]);

    var genericArgs = [typeConstructor];
    if (deferredBaseClass != null && deferredBaseClass.isNotEmpty) {
      genericArgs.add(js.call('(#) => { #; }', [jsFormals, deferredBaseClass]));
    }

    var genericCall = _callHelper('generic(#)', [genericArgs]);

    var genericName = _emitTopLevelNameNoInterop(c, suffix: '\$');
    return js.statement('{ # = #; # = #(); }',
        [genericName, genericCall, _emitTopLevelName(c), genericName]);
  }

  JS.Statement _emitClassStatement(Class c, JS.Expression className,
      JS.Expression heritage, List<JS.Method> methods) {
    var name = getLocalClassName(c);
    var classExpr =
        new JS.ClassExpression(new JS.Identifier(name), heritage, methods);
    if (c.typeParameters.isNotEmpty) {
      return classExpr.toStatement();
    } else {
      return js.statement('# = #;', [className, classExpr]);
    }
  }

  void _defineClass(Class c, JS.Expression className, List<JS.Method> methods,
      List<JS.Statement> body, List<JS.Statement> deferredSupertypes) {
    if (c == coreTypes.objectClass) {
      body.add(_emitClassStatement(c, className, null, methods));
      return;
    }

    JS.Expression emitDeferredType(DartType t) {
      if (t is InterfaceType && t.typeArguments.isNotEmpty) {
        return _emitGenericClassType(t, t.typeArguments.map(emitDeferredType));
      }
      return _emitType(t);
    }

    bool shouldDefer(InterfaceType t) {
      var visited = new Set<DartType>();
      bool defer(DartType t) {
        if (t is InterfaceType) {
          var tc = t.classNode;
          if (c == tc) return true;
          if (tc == coreTypes.objectClass || !visited.add(t)) return false;
          if (t.typeArguments.any(defer)) return true;
          var mixin = tc.mixedInType;
          return mixin != null && defer(mixin.asInterfaceType) ||
              defer(tc.supertype.asInterfaceType);
        }
        if (t is TypedefType) {
          return t.typeArguments.any(defer);
        }
        if (t is FunctionType) {
          return defer(t.returnType) ||
              t.positionalParameters.any(defer) ||
              t.namedParameters.any((np) => defer(np.type)) ||
              t.typeParameters.any((tp) => defer(tp.bound));
        }
        return false;
      }

      return defer(t);
    }

    emitClassRef(InterfaceType t) {
      // TODO(jmesserly): investigate this. It seems like `lazyJSType` is
      // invalid for use in an `extends` clause, hence this workaround.
      return _emitJSInterop(t.classNode) ?? visitInterfaceType(t);
    }

    getBaseClass(int count) {
      var base = emitDeferredType(c.thisType);
      while (--count >= 0) {
        base = js.call('#.__proto__', [base]);
      }
      return base;
    }

    var mixins = <InterfaceType>[];
    var mixedInType = c.mixedInType;
    var superclass = c.superclass;
    var supertype = c.supertype.asInterfaceType;
    if (mixedInType != null) {
      mixins.add(mixedInType.asInterfaceType);
      for (;
          superclass.isSyntheticMixinImplementation;
          superclass = superclass.superclass) {
        mixins.add(hierarchy
            .getClassAsInstanceOf(c, superclass.mixedInClass)
            .asInterfaceType);
      }
      if (mixins.length > 1) mixins = mixins.reversed.toList();
      supertype = hierarchy.getClassAsInstanceOf(c, superclass).asInterfaceType;
    }

    var hasUnnamedSuper = _hasUnnamedConstructor(superclass);
    var isCallable = isCallableClass(c);

    void emitMixinConstructors(JS.Expression className, InterfaceType mixin) {
      JS.Statement mixinCtor;
      if (_hasUnnamedConstructor(mixin.classNode)) {
        mixinCtor = js.statement('#.#.call(this);', [
          emitClassRef(mixin),
          _usesMixinNew(mixin.classNode)
              ? _callHelper('mixinNew')
              : _constructorName('')
        ]);
      }

      for (var ctor in superclass.constructors) {
        var jsParams = _emitFormalParameters(ctor.function);
        var ctorBody = <JS.Statement>[];
        if (mixinCtor != null) ctorBody.add(mixinCtor);
        var name = ctor.name.name;
        if (name != '' || hasUnnamedSuper) {
          ctorBody.add(_emitSuperConstructorCall(className, name, jsParams));
        }
        body.add(_addConstructorToClass(
            className,
            name,
            _finishConstructorFunction(
                jsParams, new JS.Block(ctorBody), isCallable)));
      }
    }

    var savedTopLevelClass = _classEmittingTopLevel;
    _classEmittingTopLevel = c;

    // Unroll mixins.
    if (shouldDefer(supertype)) {
      deferredSupertypes.add(_callHelperStatement('setBaseClass(#, #)', [
        getBaseClass(isMixinAliasClass(c) ? 0 : mixins.length),
        emitDeferredType(supertype),
      ]));
      supertype = supertype.classNode.rawType;
    }
    var baseClass = emitClassRef(supertype);

    if (isMixinAliasClass(c)) {
      // Given `class C = Object with M [implements I1, I2 ...];`
      // The resulting class C should work as a mixin.
      //
      // TODO(jmesserly): is there any way to merge this with the other mixin
      // code paths, or will these always need special handling?
      body.add(_emitClassStatement(c, className, baseClass, []));

      var m = c.mixedInType.asInterfaceType;
      bool deferMixin = shouldDefer(m);
      var mixinBody = deferMixin ? deferredSupertypes : body;
      var mixinClass = deferMixin ? emitDeferredType(m) : emitClassRef(m);
      var classExpr = deferMixin ? getBaseClass(0) : className;

      mixinBody.add(
          _callHelperStatement('mixinMembers(#, #)', [classExpr, mixinClass]));

      _classEmittingTopLevel = savedTopLevelClass;

      if (methods.isNotEmpty) {
        // However we may need to add some methods to this class that call
        // `super` such as covariance checks.
        //
        // We do this with the following pattern:
        //
        //     mixinMembers(C, class C$ extends M { <methods>  });
        mixinBody.add(_callHelperStatement('mixinMembers(#, #)', [
          classExpr,
          new JS.ClassExpression(
              new JS.TemporaryId(getLocalClassName(c)), mixinClass, methods)
        ]));
      }

      emitMixinConstructors(className, m);
      return;
    }

    // TODO(jmesserly): we need to unroll kernel mixins because the synthetic
    // classes lack required synthetic members, such as constructors.
    //
    // Also, we need to generate one extra level of nesting for alias classes.
    for (int i = 0; i < mixins.length; i++) {
      var m = mixins[i];
      var mixinName =
          getLocalClassName(superclass) + '_' + getLocalClassName(m.classNode);
      var mixinId = new JS.TemporaryId(mixinName + '\$');
      // Bind the mixin class to a name to workaround a V8 bug with es6 classes
      // and anonymous function names.
      // TODO(leafp:) Eliminate this once the bug is fixed:
      // https://bugs.chromium.org/p/v8/issues/detail?id=7069
      body.add(js.statement("const # = #", [
        mixinId,
        new JS.ClassExpression(new JS.TemporaryId(mixinName), baseClass, [])
      ]));

      emitMixinConstructors(mixinId, m);
      hasUnnamedSuper = hasUnnamedSuper || _hasUnnamedConstructor(m.classNode);

      if (shouldDefer(m)) {
        deferredSupertypes.add(_callHelperStatement('mixinMembers(#, #)',
            [getBaseClass(mixins.length - i), emitDeferredType(m)]));
      } else {
        body.add(_callHelperStatement(
            'mixinMembers(#, #)', [mixinId, emitClassRef(m)]));
      }

      baseClass = mixinId;
    }

    _classEmittingTopLevel = savedTopLevelClass;

    body.add(_emitClassStatement(c, className, baseClass, methods));
  }

  /// Defines all constructors for this class as ES5 constructors.
  List<JS.Statement> _defineConstructors(Class c, JS.Expression className) {
    var isCallable = isCallableClass(c);

    var body = <JS.Statement>[];
    if (isCallable) {
      // Our class instances will have JS `typeof this == "function"`,
      // so make sure to attach the runtime type information the same way
      // we would do it for function types.
      body.add(js.statement('#.prototype[#] = #;',
          [className, _callHelper('_runtimeType'), className]));
    }

    if (c.isSyntheticMixinImplementation || isMixinAliasClass(c)) {
      // We already handled this when we defined the class.
      return body;
    }

    addConstructor(String name, JS.Expression jsCtor) {
      body.add(_addConstructorToClass(className, name, jsCtor));
    }

    var fields = c.fields;
    for (var ctor in c.constructors) {
      if (ctor.isExternal) continue;
      addConstructor(ctor.name.name,
          _emitConstructor(ctor, fields, isCallable, className));
    }

    // If classElement has only factory constructors, and it can be mixed in,
    // then we need to emit a special hidden default constructor for use by
    // mixins.
    if (_usesMixinNew(c)) {
      body.add(
          js.statement('(#[#] = function() { # }).prototype = #.prototype;', [
        className,
        _callHelper('mixinNew'),
        [_initializeFields(fields)],
        className
      ]));
    }

    return body;
  }

  JS.Statement _emitClassTypeTests(
      Class c, JS.Expression className, List<JS.Statement> body) {
    JS.Expression getInterfaceSymbol(Class interface) {
      var library = interface.enclosingLibrary;
      if (library == coreTypes.coreLibrary ||
          library == coreTypes.asyncLibrary) {
        switch (interface.name) {
          case 'List':
          case 'Map':
          case 'Iterable':
          case 'Future':
          case 'Stream':
          case 'StreamSubscription':
            return _callHelper('is' + interface.name);
        }
      }
      return null;
    }

    void markSubtypeOf(JS.Expression testSymbol) {
      body.add(js.statement('#.prototype[#] = true', [className, testSymbol]));
    }

    for (var iface in c.implementedTypes) {
      var prop = getInterfaceSymbol(iface.classNode);
      if (prop != null) markSubtypeOf(prop);
    }

    // TODO(jmesserly): share these hand coded type checks with the old back
    // end, perhaps by factoring them into a common file, or move them to be
    // static methdos in the SDK. (Or wait until we delete the old back end.)
    if (c.enclosingLibrary == coreTypes.coreLibrary) {
      if (c == coreTypes.objectClass) {
        // Everything is an Object.
        body.add(js.statement(
            '#.is = function is_Object(o) { return true; }', [className]));
        body.add(js.statement(
            '#.as = function as_Object(o) { return o; }', [className]));
        body.add(js.statement(
            '#._check = function check_Object(o) { return o; }', [className]));
        return null;
      }
      if (c == coreTypes.stringClass) {
        body.add(js.statement(
            '#.is = function is_String(o) { return typeof o == "string"; }',
            className));
        body.add(js.statement(
            '#.as = function as_String(o) {'
            '  if (typeof o == "string" || o == null) return o;'
            '  return #.as(o, #, false);'
            '}',
            [className, _runtimeModule, className]));
        body.add(js.statement(
            '#._check = function check_String(o) {'
            '  if (typeof o == "string" || o == null) return o;'
            '  return #.as(o, #, true);'
            '}',
            [className, _runtimeModule, className]));
        return null;
      }
      if (c == coreTypes.functionClass) {
        body.add(js.statement(
            '#.is = function is_Function(o) { return typeof o == "function"; }',
            className));
        body.add(js.statement(
            '#.as = function as_Function(o) {'
            '  if (typeof o == "function" || o == null) return o;'
            '  return #.as(o, #, false);'
            '}',
            [className, _runtimeModule, className]));
        body.add(js.statement(
            '#._check = function check_String(o) {'
            '  if (typeof o == "function" || o == null) return o;'
            '  return #.as(o, #, true);'
            '}',
            [className, _runtimeModule, className]));
        return null;
      }
      if (c == coreTypes.intClass) {
        body.add(js.statement(
            '#.is = function is_int(o) {'
            '  return typeof o == "number" && Math.floor(o) == o;'
            '}',
            className));
        body.add(js.statement(
            '#.as = function as_int(o) {'
            '  if ((typeof o == "number" && Math.floor(o) == o) || o == null)'
            '    return o;'
            '  return #.as(o, #, false);'
            '}',
            [className, _runtimeModule, className]));
        body.add(js.statement(
            '#._check = function check_int(o) {'
            '  if ((typeof o == "number" && Math.floor(o) == o) || o == null)'
            '    return o;'
            '  return #.as(o, #, true);'
            '}',
            [className, _runtimeModule, className]));
        return null;
      }
      if (c == coreTypes.nullClass) {
        body.add(js.statement(
            '#.is = function is_Null(o) { return o == null; }', className));
        body.add(js.statement(
            '#.as = function as_Null(o) {'
            '  if (o == null) return o;'
            '  return #.as(o, #, false);'
            '}',
            [className, _runtimeModule, className]));
        body.add(js.statement(
            '#._check = function check_Null(o) {'
            '  if (o == null) return o;'
            '  return #.as(o, #, true);'
            '}',
            [className, _runtimeModule, className]));
        return null;
      }
      if (c == coreTypes.numClass || c == coreTypes.doubleClass) {
        body.add(js.statement(
            '#.is = function is_num(o) { return typeof o == "number"; }',
            className));
        body.add(js.statement(
            '#.as = function as_num(o) {'
            '  if (typeof o == "number" || o == null) return o;'
            '  return #.as(o, #, false);'
            '}',
            [className, _runtimeModule, className]));
        body.add(js.statement(
            '#._check = function check_num(o) {'
            '  if (typeof o == "number" || o == null) return o;'
            '  return #.as(o, #, true);'
            '}',
            [className, _runtimeModule, className]));
        return null;
      }
      if (c == coreTypes.boolClass) {
        body.add(js.statement(
            '#.is = function is_bool(o) { return o === true || o === false; }',
            className));
        body.add(js.statement(
            '#.as = function as_bool(o) {'
            '  if (o === true || o === false || o == null) return o;'
            '  return #.as(o, #, false);'
            '}',
            [className, _runtimeModule, className]));
        body.add(js.statement(
            '#._check = function check_bool(o) {'
            '  if (o === true || o === false || o == null) return o;'
            '  return #.as(o, #, true);'
            '}',
            [className, _runtimeModule, className]));
        return null;
      }
    }
    if (c.enclosingLibrary == coreTypes.asyncLibrary) {
      if (c == coreTypes.futureOrClass) {
        var typeParam = new TypeParameterType(c.typeParameters[0]);
        var typeT = visitTypeParameterType(typeParam);
        var futureOfT = visitInterfaceType(
            new InterfaceType(coreTypes.futureClass, [typeParam]));
        body.add(js.statement('''
            #.is = function is_FutureOr(o) {
              return #.is(o) || #.is(o);
            }
            ''', [className, typeT, futureOfT]));
        // TODO(jmesserly): remove the fallback to `dart.as`. It's only for the
        // _ignoreTypeFailure logic.
        body.add(js.statement('''
            #.as = function as_FutureOr(o) {
              if (o == null || #.is(o) || #.is(o)) return o;
              return #.as(o, this, false);
            }
            ''', [className, typeT, futureOfT, _runtimeModule]));
        body.add(js.statement('''
            #._check = function check_FutureOr(o) {
              if (o == null || #.is(o) || #.is(o)) return o;
              return #.as(o, this, true);
            }
            ''', [className, typeT, futureOfT, _runtimeModule]));
        return null;
      }
    }

    body.add(_callHelperStatement('addTypeTests(#);', [className]));

    if (c.typeParameters.isEmpty) return null;

    // For generics, testing against the default instantiation is common,
    // so optimize that.
    var isClassSymbol = getInterfaceSymbol(c);
    if (isClassSymbol == null) {
      // TODO(jmesserly): we could export these symbols, if we want to mark
      // implemented interfaces for user-defined classes.
      var id = new JS.TemporaryId("_is_${getLocalClassName(c)}_default");
      _moduleItems.add(
          js.statement('const # = Symbol(#);', [id, js.string(id.name, "'")]));
      isClassSymbol = id;
    }
    // Marking every generic type instantiation as a subtype of its default
    // instantiation.
    markSubtypeOf(isClassSymbol);

    // Define the type tests on the default instantiation to check for that
    // marker.
    var defaultInst = _emitTopLevelName(c);

    // Return this `addTypeTests` call so we can emit it outside of the generic
    // type parameter scope.
    return _callHelperStatement(
        'addTypeTests(#, #);', [defaultInst, isClassSymbol]);
  }

  void _emitSymbols(Iterable<JS.TemporaryId> vars, List<JS.ModuleItem> body) {
    for (var id in vars) {
      body.add(js.statement('const # = Symbol(#)', [id, js.string(id.name)]));
    }
  }

  void _emitSuperHelperSymbols(List<JS.Statement> body) {
    _emitSymbols(
        _superHelpers.values.map((m) => m.name as JS.TemporaryId), body);
    _superHelpers.clear();
  }

  /// Emits static fields for a class, and initialize them eagerly if possible,
  /// otherwise define them as lazy properties.
  void _emitStaticFields(Class c, List<JS.Statement> body) {
    var fields = c.fields.where((f) => f.isStatic).toList();
    if (c.isEnum) {
      // We know enum fields can be safely emitted as const fields, as long
      // as the `values` field is emitted last.
      var classRef = _emitTopLevelName(c);
      var valueField = fields.firstWhere((f) => f.name.name == 'values');
      fields.remove(valueField);
      fields.add(valueField);
      for (var f in fields) {
        assert(f.isConst);
        body.add(_defineValueOnClass(
                classRef,
                _emitStaticMemberName(f.name.name),
                _visitInitializer(f.initializer, f.annotations))
            .toStatement());
      }
    } else if (fields.isNotEmpty) {
      body.add(_emitLazyFields(_emitTopLevelName(c), fields));
    }
  }

  void _emitClassMetadata(List<Expression> metadata, JS.Expression className,
      List<JS.Statement> body) {
    // Metadata
    if (emitMetadata && metadata.isNotEmpty) {
      body.add(js.statement('#[#.metadata] = #;', [
        className,
        _runtimeModule,
        _arrowFunctionWithLetScope(() => new JS.ArrayInitializer(
            metadata.map(_instantiateAnnotation).toList()))
      ]));
    }
  }

  /// Ensure `dartx.` symbols we will use are present.
  void _initExtensionSymbols(Class c) {
    if (_extensionTypes.hasNativeSubtype(c) || c == coreTypes.objectClass) {
      for (var m in c.procedures) {
        if (!m.isAbstract && !m.isStatic && !m.name.isPrivate) {
          _declareMemberName(m, useExtension: true);
        }
      }
    }
  }

  /// If a concrete class implements one of our extensions, we might need to
  /// add forwarders.
  void _defineExtensionMembers(
      JS.Expression className, List<JS.Statement> body) {
    void emitExtensions(String helperName, Iterable<String> extensions) {
      if (extensions.isEmpty) return;

      var names = extensions
          .map((e) => _propertyName(JS.memberNameForDartMember(e)))
          .toList();
      body.add(js.statement('#.#(#, #);', [
        _runtimeModule,
        helperName,
        className,
        new JS.ArrayInitializer(names, multiline: names.length > 4)
      ]));
    }

    var props = _classProperties;
    emitExtensions('defineExtensionMethods', props.extensionMethods);
    emitExtensions('defineExtensionAccessors', props.extensionAccessors);
  }

  /// Emit the signature on the class recording the runtime type information
  void _emitClassSignature(
      Class c, JS.Expression className, List<JS.Statement> body) {
    if (c.implementedTypes.isNotEmpty) {
      body.add(js.statement('#[#.implements] = () => [#];', [
        className,
        _runtimeModule,
        c.implementedTypes.map((i) => _emitType(i.asInterfaceType))
      ]));
    }

    void emitSignature(String name, List<JS.Property> elements) {
      if (elements.isEmpty) return;

      if (!name.startsWith('Static')) {
        var proto = c == coreTypes.objectClass
            ? js.call('Object.create(null)')
            : _callHelper('get${name}s(#.__proto__)', [className]);
        elements.insert(0, new JS.Property(_propertyName('__proto__'), proto));
      }
      body.add(_callHelperStatement('set${name}Signature(#, () => #)', [
        className,
        new JS.ObjectInitializer(elements, multiline: elements.length > 1)
      ]));
    }

    var extMembers = _classProperties.extensionMethods;
    var staticMethods = <JS.Property>[];
    var instanceMethods = <JS.Property>[];
    var staticGetters = <JS.Property>[];
    var instanceGetters = <JS.Property>[];
    var staticSetters = <JS.Property>[];
    var instanceSetters = <JS.Property>[];
    List<JS.Property> getSignatureList(Procedure p) {
      if (p.isStatic) {
        if (p.isGetter) {
          return staticGetters;
        } else if (p.isSetter) {
          return staticSetters;
        } else {
          return staticMethods;
        }
      } else {
        if (p.isGetter) {
          return instanceGetters;
        } else if (p.isSetter) {
          return instanceSetters;
        } else {
          return instanceMethods;
        }
      }
    }

    for (var member in c.procedures) {
      if (member.isAbstract) continue;

      // Static getters/setters cannot be called with dynamic dispatch, nor
      // can they be torn off.
      // TODO(jmesserly): can we attach static method type info at the tearoff
      // point, and avoid saving the information otherwise? Same trick would
      // work for top-level functions.
      if (!emitMetadata && member.isAccessor && member.isStatic) {
        continue;
      }

      var name = member.name.name;
      var reifiedType = _getMemberRuntimeType(member);

      // Don't add redundant signatures for inherited methods whose signature
      // did not change.  If we are not overriding, or if the thing we are
      // overriding has a different reified type from ourselves, we must
      // emit a signature on this class.  Otherwise we will inherit the
      // signature from the superclass.
      var memberOverride = c.superclass != null
          ? hierarchy.getDispatchTarget(c.superclass, member.name,
              setter: member.isSetter)
          : null;

      var needsSignature = memberOverride == null ||
          reifiedType !=
              Substitution
                  .fromSupertype(hierarchy.getClassAsInstanceOf(
                      c, memberOverride.enclosingClass))
                  .substituteType(_getMemberRuntimeType(memberOverride));

      if (needsSignature) {
        var type = _emitAnnotatedFunctionType(reifiedType, member.annotations,
            function: member.function);
        var property = new JS.Property(_declareMemberName(member), type);
        var signatures = getSignatureList(member);
        signatures.add(property);
        if (!member.isStatic && extMembers.contains(name)) {
          signatures.add(new JS.Property(
              _declareMemberName(member, useExtension: true), type));
        }
      }
    }

    emitSignature('Method', instanceMethods);
    emitSignature('StaticMethod', staticMethods);
    emitSignature('Getter', instanceGetters);
    emitSignature('Setter', instanceSetters);
    emitSignature('StaticGetter', staticGetters);
    emitSignature('StaticSetter', staticSetters);

    var instanceFields = <JS.Property>[];
    var staticFields = <JS.Property>[];
    for (var field in c.fields) {
      // Only instance fields need to be saved for dynamic dispatch.
      var isStatic = field.isStatic;
      if (!emitMetadata && isStatic) continue;

      var memberName = _declareMemberName(field);
      var fieldSig = _emitFieldSignature(field.type,
          metadata: field.annotations, isFinal: field.isFinal);
      (isStatic ? staticFields : instanceFields)
          .add(new JS.Property(memberName, fieldSig));
    }
    emitSignature('Field', instanceFields);
    emitSignature('StaticField', staticFields);

    var constructors = <JS.Property>[];
    if (emitMetadata) {
      for (var ctor in c.constructors) {
        var memberName = _constructorName(ctor.name.name);
        var type = _emitAnnotatedFunctionType(
            ctor.function.functionType.withoutTypeParameters, ctor.annotations,
            function: ctor.function, nameType: false, definite: true);
        constructors.add(new JS.Property(memberName, type));
      }
    }
    emitSignature('Constructor', constructors);

    // Add static property dart._runtimeType to Object.
    // All other Dart classes will (statically) inherit this property.
    if (c == coreTypes.objectClass) {
      body.add(_callHelperStatement('tagComputed(#, () => #.#);',
          [className, emitLibraryName(coreTypes.coreLibrary), 'Type']));
    }
  }

  JS.Expression _emitFieldSignature(DartType type,
      {List<Expression> metadata, bool isFinal: true}) {
    var args = [_emitType(type)];
    if (emitMetadata && metadata != null && metadata.isNotEmpty) {
      args.add(new JS.ArrayInitializer(
          metadata.map(_instantiateAnnotation).toList()));
    }
    return _callHelper(isFinal ? 'finalFieldType(#)' : 'fieldType(#)', [args]);
  }

  FunctionType _getMemberRuntimeType(Member member) {
    var f = member.function;
    if (f == null) {
      assert(member is Field);
      return new FunctionType([], member.getterType);
    }

    if (!f.positionalParameters.any(isCovariant) &&
        !f.namedParameters.any(isCovariant)) {
      return f.functionType;
    }

    reifyParameter(VariableDeclaration p) =>
        isCovariant(p) ? coreTypes.objectClass.thisType : p.type;
    reifyNamedParameter(VariableDeclaration p) =>
        new NamedType(p.name, reifyParameter(p));

    // TODO(jmesserly): do covariant type parameter bounds also need to be
    // reified as `Object`?
    return new FunctionType(
        f.positionalParameters.map(reifyParameter).toList(), f.returnType,
        namedParameters: f.namedParameters.map(reifyNamedParameter).toList()
          ..sort(),
        typeParameters: f.functionType.typeParameters,
        requiredParameterCount: f.requiredParameterCount);
  }

  JS.Expression _emitConstructor(Constructor node, List<Field> fields,
      bool isCallable, JS.Expression className) {
    var params = _emitFormalParameters(node.function);
    var body = _withCurrentFunction(
        node.function,
        () => _superDisallowed(
            () => _emitConstructorBody(node, fields, className)));

    return _finishConstructorFunction(params, new JS.Block(body), isCallable)
      ..sourceInformation = _nodeEnd(node.fileEndOffset) ??
          _nodeEnd(node.enclosingClass.fileEndOffset);
  }

  List<JS.Statement> _emitConstructorBody(
      Constructor node, List<Field> fields, JS.Expression className) {
    var cls = node.enclosingClass;

    // Generate optional/named argument value assignment. These can not have
    // side effects, and may be used by the constructor's initializers, so it's
    // nice to do them first.
    // Also for const constructors we need to ensure default values are
    // available for use by top-level constant initializers.
    var fn = node.function;
    var body = _emitArgumentInitializers(fn);

    // Redirecting constructors: these are not allowed to have initializers,
    // and the redirecting ctor invocation runs before field initializers.
    var redirectCall = node.initializers
            .firstWhere((i) => i is RedirectingInitializer, orElse: () => null)
        as RedirectingInitializer;

    if (redirectCall != null) {
      body.add(_emitRedirectingConstructor(redirectCall, className));
      return body;
    }

    // Generate field initializers.
    // These are expanded into each non-redirecting constructor.
    // In the future we may want to create an initializer function if we have
    // multiple constructors, but it needs to be balanced against readability.
    body.add(_initializeFields(fields, node));

    // If no superinitializer is provided, an implicit superinitializer of the
    // form `super()` is added at the end of the initializer list, unless the
    // enclosing class is class Object.
    var superCall = node.initializers.firstWhere((i) => i is SuperInitializer,
        orElse: () => null) as SuperInitializer;
    var jsSuper = _emitSuperConstructorCallIfNeeded(cls, className, superCall);
    if (jsSuper != null) {
      body.add(jsSuper..sourceInformation = _nodeStart(superCall));
    }

    body.add(_emitFunctionScopedBody(fn));
    return body;
  }

  JS.Expression _constructorName(String name) {
    if (name == '') {
      // Default constructors (factory or not) use `new` as their name.
      return _propertyName('new');
    }
    return _emitStaticMemberName(name);
  }

  JS.Statement _emitRedirectingConstructor(
      RedirectingInitializer node, JS.Expression className) {
    var ctor = node.target;
    // We can't dispatch to the constructor with `this.new` as that might hit a
    // derived class constructor with the same name.
    return js.statement('#.#.call(this, #);', [
      className,
      _constructorName(ctor.name.name),
      _emitArgumentList(node.arguments)
    ]);
  }

  JS.Statement _emitSuperConstructorCallIfNeeded(
      Class c, JS.Expression className,
      [SuperInitializer superInit]) {
    if (c == coreTypes.objectClass) return null;

    Constructor ctor;
    List<JS.Expression> args;
    if (superInit == null) {
      ctor = unnamedConstructor(c.superclass);
      args = [];
    } else {
      ctor = superInit.target;
      args = _emitArgumentList(superInit.arguments);
    }
    // We can skip the super call if it's empty. Most commonly this happens for
    // things that extend Object, and don't have any field initializers or their
    // own default constructor.
    if (ctor.name.name == '' && !_hasUnnamedSuperConstructor(c)) {
      return null;
    }
    return _emitSuperConstructorCall(className, ctor.name.name, args);
  }

  JS.Statement _emitSuperConstructorCall(
      JS.Expression className, String name, List<JS.Expression> args) {
    return js.statement('#.__proto__.#.call(this, #);',
        [className, _constructorName(name), args ?? []]);
  }

  bool _hasUnnamedSuperConstructor(Class c) {
    if (c == null) return false;
    return _hasUnnamedConstructor(c.superclass) ||
        _hasUnnamedConstructor(c.mixedInClass);
  }

  bool _hasUnnamedConstructor(Class c) {
    if (c == null || c == coreTypes.objectClass) return false;
    var ctor = unnamedConstructor(c);
    if (ctor != null && !ctor.isSynthetic) return true;
    if (c.fields.any((f) => !f.isStatic)) return true;
    return _hasUnnamedSuperConstructor(c);
  }

  JS.Expression _finishConstructorFunction(
      List<JS.Parameter> params, JS.Block body, bool isCallable) {
    // We consider a class callable if it inherits from anything with a `call`
    // method. As a result, we can know the callable JS function was created
    // at the first constructor that was hit.
    if (!isCallable) return new JS.Fun(params, body);
    return js.call(r'''function callableClass(#) {
          if (typeof this !== "function") {
            function self(...args) {
              return self.call.apply(self, args);
            }
            self.__proto__ = this.__proto__;
            callableClass.call(self, #);
            return self;
          }
          #
        }''', [params, params, body]);
  }

  /// Initialize fields. They follow the sequence:
  ///
  ///   1. field declaration initializer if non-const,
  ///   2. field initializing parameters,
  ///   3. constructor field initializers,
  ///   4. initialize fields not covered in 1-3
  JS.Statement _initializeFields(List<Field> fields, [Constructor ctor]) {
    // Run field initializers if they can have side-effects.
    Set<Field> ctorFields;
    if (ctor != null) {
      ctorFields = ctor.initializers
          .map((c) => c is FieldInitializer ? c.field : null)
          .toSet()
            ..remove(null);
    }

    var body = <JS.Statement>[];
    emitFieldInit(Field f, Expression initializer, TreeNode hoverInfo) {
      var access = _classProperties.virtualFields[f] ?? _declareMemberName(f);
      var jsInit = _visitInitializer(initializer, f.annotations);
      body.add(jsInit
          .toAssignExpression(js.call('this.#', [access])
            ..sourceInformation = _nodeStart(hoverInfo))
          .toStatement());
    }

    for (var f in fields) {
      if (f.isStatic) continue;
      var init = f.initializer;
      if (ctorFields != null &&
          ctorFields.contains(f) &&
          (init == null || _constants.isConstant(init))) {
        continue;
      }
      emitFieldInit(f, init, f);
    }

    // Run constructor field initializers such as `: foo = bar.baz`
    if (ctor != null) {
      for (var init in ctor.initializers) {
        if (init is FieldInitializer) {
          emitFieldInit(init.field, init.value, init);
        } else if (init is LocalInitializer) {
          body.add(visitVariableDeclaration(init.variable));
        } else if (init is AssertInitializer) {
          body.add(visitAssertStatement(init.statement));
        }
      }
    }

    return JS.Statement.from(body);
  }

  JS.Expression _visitInitializer(
      Expression init, List<Expression> annotations) {
    // explicitly initialize to null, to avoid getting `undefined`.
    // TODO(jmesserly): do this only for vars that aren't definitely assigned.
    if (init == null) return new JS.LiteralNull();
    return _annotatedNullCheck(annotations)
        ? notNull(init)
        : _visitExpression(init);
  }

  JS.Expression notNull(Expression expr) {
    if (expr == null) return null;
    var jsExpr = _visitExpression(expr);
    if (!isNullable(expr)) return jsExpr;
    return _callHelper('notNull(#)', jsExpr);
  }

  /// If the class has only factory constructors, and it can be mixed in,
  /// then we need to emit a special hidden default constructor for use by
  /// mixins.
  bool _usesMixinNew(Class mixin) {
    return mixin.superclass?.superclass == null &&
        mixin.constructors.every((c) => c.isExternal);
  }

  JS.Statement _addConstructorToClass(
      JS.Expression className, String name, JS.Expression jsCtor) {
    jsCtor = _defineValueOnClass(className, _constructorName(name), jsCtor);
    return js.statement('#.prototype = #.prototype;', [jsCtor, className]);
  }

  JS.Expression _defineValueOnClass(
      JS.Expression className, JS.Expression name, JS.Expression value) {
    var args = [className, name, value];
    if (name is JS.LiteralString &&
        JS.invalidStaticFieldName(name.valueWithoutQuotes)) {
      return _callHelper('defineValue(#, #, #)', args);
    }
    return js.call('#.# = #', args);
  }

  List<JS.Method> _emitClassMethods(Class c) {
    var virtualFields = _classProperties.virtualFields;

    var jsMethods = <JS.Method>[];
    bool hasJsPeer = findAnnotation(c, isJsPeerInterface) != null;
    bool hasIterator = false;

    if (c == coreTypes.objectClass) {
      // Dart does not use ES6 constructors.
      // Add an error to catch any invalid usage.
      jsMethods.add(
          new JS.Method(_propertyName('constructor'), js.fun(r'''function() {
                  throw Error("use `new " + #.typeName(#.getReifiedType(this)) +
                      ".new(...)` to create a Dart object");
              }''', [_runtimeModule, _runtimeModule])));
    }

    for (var m in c.fields) {
      if (_extensionTypes.isNativeClass(c)) {
        jsMethods.addAll(_emitNativeFieldAccessors(m));
        continue;
      }
      if (m.isStatic) continue;
      if (virtualFields.containsKey(m)) {
        jsMethods.addAll(_emitVirtualFieldAccessor(m));
      }
    }

    var getters = new Map<String, Procedure>();
    var setters = new Map<String, Procedure>();
    for (var m in c.procedures) {
      if (m.isAbstract) continue;
      if (m.isGetter) {
        getters[m.name.name] = m;
      } else if (m.isSetter) {
        setters[m.name.name] = m;
      }
    }

    for (var m in c.procedures) {
      if (m.isForwardingStub) {
        // TODO(jmesserly): is there any other kind of forwarding stub?
        jsMethods.addAll(_emitCovarianceCheckStub(m));
      } else if (m.isFactory) {
        jsMethods.add(_emitFactoryConstructor(m));
      } else if (m.isAccessor) {
        jsMethods.add(_emitMethodDeclaration(m));
        jsMethods.add(_emitSuperAccessorWrapper(m, getters, setters));
        if (!hasJsPeer && m.isGetter && m.name.name == 'iterator') {
          hasIterator = true;
          jsMethods.add(_emitIterable(c));
        }
      } else {
        jsMethods.add(_emitMethodDeclaration(m));
      }
    }

    for (Member m in _classProperties.mockMembers.values) {
      _addMockMembers(m, c, jsMethods);
    }

    // If the type doesn't have an `iterator`, but claims to implement Iterable,
    // we inject the adaptor method here, as it's less code size to put the
    // helper on a parent class. This pattern is common in the core libraries
    // (e.g. IterableMixin<E> and IterableBase<E>).
    //
    // (We could do this same optimization for any interface with an `iterator`
    // method, but that's more expensive to check for, so it doesn't seem worth
    // it. The above case for an explicit `iterator` method will catch those.)
    if (!hasJsPeer && !hasIterator) {
      jsMethods.add(_emitIterable(c));
    }

    // Add all of the super helper methods
    jsMethods.addAll(_superHelpers.values);

    return jsMethods.where((m) => m != null).toList();
  }

  /// Emits a method, getter, or setter.
  JS.Method _emitMethodDeclaration(Procedure member) {
    if (member.isAbstract) {
      return null;
    }

    JS.Fun fn;
    if (member.isExternal) {
      if (member.isStatic) {
        // TODO(vsm): Do we need to handle this case?
        return null;
      }
      fn = _emitNativeFunctionBody(member);
    } else {
      fn = _emitFunction(member.function, member.name.name);
    }

    return new JS.Method(_declareMemberName(member), fn,
        isGetter: member.isGetter,
        isSetter: member.isSetter,
        isStatic: member.isStatic)
      ..sourceInformation = _nodeEnd(member.fileEndOffset);
  }

  JS.Fun _emitNativeFunctionBody(Procedure node) {
    String name = getAnnotationName(node, isJSAnnotation) ?? node.name.name;
    if (node.isGetter) {
      return new JS.Fun([], js.block('{ return this.#; }', [name]));
    } else if (node.isSetter) {
      var params = _emitFormalParameters(node.function);
      return new JS.Fun(
          params, js.block('{ this.# = #; }', [name, params.last]));
    } else {
      return js.fun(
          'function (...args) { return this.#.apply(this, args); }', name);
    }
  }

  List<JS.Method> _emitCovarianceCheckStub(Procedure member) {
    // TODO(jmesserly): kernel stubs have a few problems:
    // - they're generated even when there is no concrete super member
    // - the stub parameter types don't match the types we need to check to
    //   ensure soundness of the super member, so we must lookup the super
    //   member and determine checks ourselves.
    // - it generates getter stubs, but these are not used
    if (member.isGetter) return [];

    var enclosingClass = member.enclosingClass;
    var superMember = member.forwardingStubSuperTarget ??
        member.forwardingStubInterfaceTarget;

    if (superMember == null) return [];

    var superSubstition = Substitution.fromSupertype(hierarchy
        .getClassAsInstanceOf(enclosingClass, superMember.enclosingClass));

    var name = _declareMemberName(member);
    if (member.isSetter) {
      if (superMember is Field && superMember.isGenericCovariantImpl ||
          superMember is Procedure &&
              isCovariant(superMember.function.positionalParameters[0])) {
        return [];
      }
      return [
        new JS.Method(
            name,
            js.fun('function(x) { return super.# = #._check(x); }', [
              name,
              _emitType(superSubstition.substituteType(superMember.setterType))
            ]),
            isSetter: true),
        new JS.Method(name, js.fun('function() { return super.#; }', [name]),
            isGetter: true)
      ];
    }
    assert(!member.isAccessor);

    var superMethodType = superSubstition
        .substituteType(superMember.function.functionType) as FunctionType;
    var function = member.function;

    var body = <JS.Statement>[];
    var typeParameters = superMethodType.typeParameters;
    _emitCovarianceBoundsCheck(typeParameters, body);

    var typeFormals = _emitTypeFormals(typeParameters);
    var jsParams = new List<JS.Parameter>.from(typeFormals);
    var positionalParameters = function.positionalParameters;
    for (var i = 0, n = positionalParameters.length; i < n; i++) {
      var param = positionalParameters[i];
      var jsParam = new JS.Identifier(param.name);
      jsParams.add(jsParam);

      if (isCovariant(param) &&
          !isCovariant(superMember.function.positionalParameters[i])) {
        var check = js.call('#._check(#)',
            [_emitType(superMethodType.positionalParameters[i]), jsParam]);
        if (i >= function.requiredParameterCount) {
          body.add(js.statement('if (# !== void 0) #;', [jsParam, check]));
        } else {
          body.add(check.toStatement());
        }
      }
    }
    var namedParameters = function.namedParameters;
    for (var param in namedParameters) {
      if (isCovariant(param) &&
          !isCovariant(superMember.function.namedParameters
              .firstWhere((n) => n.name == param.name))) {
        var name = _propertyName(param.name);
        var paramType = superMethodType.namedParameters
            .firstWhere((n) => n.name == param.name);
        body.add(js.statement('if (# in #) #._check(#.#);', [
          name,
          namedArgumentTemp,
          _emitType(paramType.type),
          namedArgumentTemp,
          name
        ]));
      }
    }

    if (namedParameters.isNotEmpty) jsParams.add(namedArgumentTemp);

    if (typeFormals.isEmpty) {
      body.add(js.statement('return super.#(#);', [name, jsParams]));
    } else {
      body.add(
          js.statement('return super.#(#)(#);', [name, typeFormals, jsParams]));
    }
    var fn = new JS.Fun(jsParams, new JS.Block(body));
    return [new JS.Method(name, fn)];
  }

  /// Emits a Dart factory constructor to a JS static method.
  JS.Method _emitFactoryConstructor(Procedure node) {
    if (isUnsupportedFactoryConstructor(node)) return null;

    return new JS.Method(
        _constructorName(node.name.name),
        new JS.Fun(_emitFormalParameters(node.function),
            _emitFunctionBody(node.function)),
        isStatic: true)
      ..sourceInformation = _nodeEnd(node.fileEndOffset);
  }

  /// Emits an expression that lets you access statics on a [type] from code.
  ///
  /// If [nameType] is true, then the type will be named.  In addition,
  /// if [hoistType] is true, then the named type will be hoisted.
  JS.Expression emitConstructorAccess(InterfaceType type) {
    return _emitJSInterop(type.classNode) ?? visitInterfaceType(type);
  }

  /// Given a class C that implements method M from interface I, but does not
  /// declare M, this will generate an implementation that forwards to
  /// noSuchMethod.
  ///
  /// For example:
  ///
  ///     class Cat {
  ///       bool eatFood(String food) => true;
  ///     }
  ///     class MockCat implements Cat {
  ///        noSuchMethod(Invocation invocation) => 3;
  ///     }
  ///
  /// It will generate an `eatFood` that looks like:
  ///
  ///     eatFood(...args) {
  ///       return core.bool.as(this.noSuchMethod(
  ///           new dart.InvocationImpl.new('eatFood', args)));
  ///     }
  ///
  /// Same technique is applied if interface I has fields, and C doesn't declare
  /// neither the fields nor the corresponding getters and setters.
  void _addMockMembers(Member member, Class c, List<JS.Method> jsMethods) {
    JS.Method implementMockMember(
        List<TypeParameter> typeParameters,
        List<VariableDeclaration> namedParameters,
        ProcedureKind mockMemberKind,
        DartType returnType) {
      assert(mockMemberKind != ProcedureKind.Factory);

      var invocationProps = <JS.Property>[];
      addProperty(String name, JS.Expression value) {
        invocationProps.add(new JS.Property(js.string(name), value));
      }

      var args = new JS.TemporaryId('args');
      var typeParams = _emitTypeFormals(typeParameters);
      var fnArgs = new List<JS.Parameter>.from(typeParams);
      JS.Expression positionalArgs;

      if (namedParameters.isNotEmpty) {
        addProperty(
            'namedArguments', _callHelper('extractNamedArgs(#)', [args]));
      }

      if (mockMemberKind != ProcedureKind.Getter &&
          mockMemberKind != ProcedureKind.Setter) {
        addProperty('isMethod', js.boolean(true));

        fnArgs.add(new JS.RestParameter(args));
        positionalArgs = args;
      } else {
        if (mockMemberKind == ProcedureKind.Getter) {
          addProperty('isGetter', js.boolean(true));

          positionalArgs = new JS.ArrayInitializer([]);
        } else if (mockMemberKind == ProcedureKind.Setter) {
          addProperty('isSetter', js.boolean(true));

          fnArgs.add(args);
          positionalArgs = new JS.ArrayInitializer([args]);
        }
      }

      if (typeParams.isNotEmpty) {
        addProperty('typeArguments', new JS.ArrayInitializer(typeParams));
      }

      var fnBody =
          js.call('this.noSuchMethod(new #.InvocationImpl.new(#, #, #))', [
        _runtimeModule,
        _declareMemberName(member),
        positionalArgs,
        new JS.ObjectInitializer(invocationProps)
      ]);

      if (!types.isTop(returnType)) {
        fnBody = js.call('#._check(#)', [_emitType(returnType), fnBody]);
      }

      var fn = new JS.Fun(fnArgs, js.block('{ return #; }', [fnBody]),
          typeParams: typeParams);

      return new JS.Method(
          _declareMemberName(member,
              useExtension: _extensionTypes.isNativeClass(c)),
          fn,
          isGetter: mockMemberKind == ProcedureKind.Getter,
          isSetter: mockMemberKind == ProcedureKind.Setter,
          isStatic: false);
    }

    if (member is Field) {
      jsMethods.add(implementMockMember(
          const <TypeParameter>[],
          const <VariableDeclaration>[],
          ProcedureKind.Getter,
          Substitution
              .fromSupertype(
                  hierarchy.getClassAsInstanceOf(c, member.enclosingClass))
              .substituteType(member.type)));
      if (!member.isFinal) {
        jsMethods.add(implementMockMember(
            const <TypeParameter>[],
            const <VariableDeclaration>[],
            ProcedureKind.Setter,
            new DynamicType()));
      }
    } else {
      Procedure procedure = member as Procedure;
      FunctionNode function = procedure.function;
      jsMethods.add(implementMockMember(
          function.typeParameters,
          function.namedParameters,
          procedure.kind,
          Substitution
              .fromSupertype(
                  hierarchy.getClassAsInstanceOf(c, member.enclosingClass))
              .substituteType(function.returnType)));
    }
  }

  /// This is called whenever a derived class needs to introduce a new field,
  /// shadowing a field or getter/setter pair on its parent.
  ///
  /// This is important because otherwise, trying to read or write the field
  /// would end up calling the getter or setter, and one of those might not even
  /// exist, resulting in a runtime error. Even if they did exist, that's the
  /// wrong behavior if a new field was declared.
  List<JS.Method> _emitVirtualFieldAccessor(Field field) {
    var virtualField = _classProperties.virtualFields[field];
    var result = <JS.Method>[];
    var name = _declareMemberName(field);

    var mocks = _classProperties.mockMembers;
    if (!mocks.containsKey(field.name.name)) {
      var getter = js.fun('function() { return this[#]; }', [virtualField]);
      result.add(new JS.Method(name, getter, isGetter: true)
        ..sourceInformation = _nodeStart(field));
    }

    if (!mocks.containsKey(field.name.name + '=')) {
      var args = field.isFinal
          ? [new JS.Super(), name]
          : [new JS.This(), virtualField];

      String jsCode;
      if (!field.isFinal && field.isGenericCovariantImpl) {
        args.add(_emitType(field.type));
        jsCode = 'function(value) { #[#] = #._check(value); }';
      } else {
        jsCode = 'function(value) { #[#] = value; }';
      }

      result.add(new JS.Method(name, js.fun(jsCode, args), isSetter: true)
        ..sourceInformation = _nodeStart(field));
    }

    return result;
  }

  /// Provide Dart getters and setters that forward to the underlying native
  /// field.  Note that the Dart names are always symbolized to avoid
  /// conflicts.  They will be installed as extension methods on the underlying
  /// native type.
  List<JS.Method> _emitNativeFieldAccessors(Field field) {
    // TODO(vsm): Can this by meta-programmed?
    // E.g., dart.nativeField(symbol, jsName)
    // Alternatively, perhaps it could be meta-programmed directly in
    // dart.registerExtensions?
    var jsMethods = <JS.Method>[];
    if (field.isStatic) return jsMethods;

    var name = getAnnotationName(field, isJSName) ?? field.name.name;
    // Generate getter
    var fn = new JS.Fun([], js.block('{ return this.#; }', [name]));
    var method = new JS.Method(_declareMemberName(field), fn, isGetter: true);
    jsMethods.add(method);

    // Generate setter
    if (!field.isFinal) {
      var value = new JS.TemporaryId('value');
      fn = new JS.Fun([value], js.block('{ this.# = #; }', [name, value]));
      method = new JS.Method(_declareMemberName(field), fn, isSetter: true);
      jsMethods.add(method);
    }

    return jsMethods;
  }

  /// Emit a getter (or setter) that simply forwards to the superclass getter
  /// (or setter).
  ///
  /// This is needed because in ES6, if you only override a getter
  /// (alternatively, a setter), then there is an implicit override of the
  /// setter (alternatively, the getter) that does nothing.
  JS.Method _emitSuperAccessorWrapper(Procedure method,
      Map<String, Procedure> getters, Map<String, Procedure> setters) {
    var name = method.name.name;
    var memberName = _declareMemberName(method);
    if (method.isGetter) {
      if (!setters.containsKey(name) &&
          _classProperties.inheritedSetters.contains(name)) {
        // Generate a setter that forwards to super.
        var fn = js.fun('function(value) { super[#] = value; }', [memberName]);
        return new JS.Method(memberName, fn, isSetter: true);
      }
    } else {
      assert(method.isSetter);
      if (!getters.containsKey(name) &&
          _classProperties.inheritedGetters.contains(name)) {
        // Generate a getter that forwards to super.
        var fn = js.fun('function() { return super[#]; }', [memberName]);
        return new JS.Method(memberName, fn, isGetter: true);
      }
    }
    return null;
  }

  /// Support for adapting dart:core Iterable to ES6 versions.
  ///
  /// This lets them use for-of loops transparently:
  /// <https://github.com/lukehoban/es6features#iterators--forof>
  ///
  /// This will return `null` if the adapter was already added on a super type,
  /// otherwise it returns the adapter code.
  // TODO(jmesserly): should we adapt `Iterator` too?
  JS.Method _emitIterable(Class c) {
    var iterable = hierarchy.getClassAsInstanceOf(c, coreTypes.iterableClass);
    if (iterable == null) return null;

    // If a parent had an `iterator` (concrete or abstract) or implements
    // Iterable, we know the adapter is already there, so we can skip it as a
    // simple code size optimization.
    var parent =
        hierarchy.getDispatchTarget(c.superclass, new Name('iterator'));
    if (parent != null) return null;

    var parentIterable =
        hierarchy.getClassAsInstanceOf(c.superclass, coreTypes.iterableClass);
    if (parentIterable != null) return null;

    if (c.enclosingLibrary.importUri.scheme == 'dart' &&
        c.procedures.any((m) => getJSExportName(m) == 'Symbol.iterator')) {
      return null;
    }

    // Otherwise, emit the adapter method, which wraps the Dart iterator in
    // an ES6 iterator.
    return new JS.Method(
        js.call('Symbol.iterator'),
        js.call('function() { return new #.JsIterator(this.#); }', [
          _runtimeModule,
          _emitMemberName('iterator', type: iterable.asInterfaceType)
        ]) as JS.Fun);
  }

  JS.Expression _instantiateAnnotation(Expression node) =>
      _visitExpression(node);

  /// Gets the JS peer for this Dart type if any, otherwise null.
  ///
  /// For example for dart:_interceptors `JSArray` this will return "Array",
  /// referring to the JavaScript built-in `Array` type.
  List<String> _getJSPeerNames(Class c) {
    var jsPeerNames = getAnnotationName(
        c,
        (a) =>
            isJsPeerInterface(a) ||
            isNativeAnnotation(a) && _extensionTypes.isNativeClass(c));
    if (c == coreTypes.objectClass) return ['Object'];
    if (jsPeerNames == null) return [];

    // Omit the special name "!nonleaf" and any future hacks starting with "!"
    var result =
        jsPeerNames.split(',').where((peer) => !peer.startsWith("!")).toList();
    return result;
  }

  void _registerExtensionType(
      Class c, String jsPeerName, List<JS.Statement> body) {
    var className = _emitTopLevelName(c);
    if (isPrimitiveType(c.rawType)) {
      body.add(_callHelperStatement(
          'definePrimitiveHashCode(#.prototype)', className));
    }
    body.add(_callHelperStatement(
        'registerExtension(#, #);', [js.string(jsPeerName), className]));
  }

  JS.Statement _emitJSType(Class c) {
    var jsTypeName = getAnnotationName(c, isJSAnnotation);
    if (jsTypeName == null || jsTypeName == c.name) return null;

    // We export the JS type as if it was a Dart type. For example this allows
    // `dom.InputElement` to actually be HTMLInputElement.
    // TODO(jmesserly): if we had the JS name on the Element, we could just
    // generate it correctly when we refer to it.
    return js.statement('# = #;', [_emitTopLevelName(c), jsTypeName]);
  }

  void _emitTypedef(Typedef t) {
    var savedUri = _currentUri;
    _currentUri = t.fileUri;
    var body = _callHelper(
        'typedef(#, () => #)', [js.string(t.name, "'"), _emitType(t.type)]);

    JS.Statement result;
    if (t.typeParameters.isNotEmpty) {
      result = _defineClassTypeArguments(
          t, t.typeParameters, js.statement('const # = #;', [t.name, body]));
    } else {
      result = js.statement('# = #;', [_emitTopLevelName(t), body]);
    }

    _currentUri = savedUri;
    _moduleItems.add(result);
  }

  /// Treat dart:_runtime fields as safe to eagerly evaluate.
  // TODO(jmesserly): it'd be nice to avoid this special case.
  JS.Statement _emitInternalSdkFields(Iterable<Field> fields) {
    var lazyFields = <Field>[];
    var savedUri = _currentUri;

    for (var field in fields) {
      // Skip our magic undefined constant.
      if (field.name.name == 'undefined') continue;

      var init = field.initializer;
      if (init == null ||
          init is BasicLiteral ||
          init is StaticInvocation && isInlineJS(init.target) ||
          init is ConstructorInvocation &&
              isSdkInternalRuntime(init.target.enclosingLibrary)) {
        _currentUri = field.fileUri;
        _moduleItems.add(js.statement('# = #;', [
          _emitTopLevelName(field),
          _visitInitializer(init, field.annotations)
        ]));
      } else {
        lazyFields.add(field);
      }
    }

    _currentUri = savedUri;
    return _emitLazyFields(emitLibraryName(_currentLibrary), lazyFields);
  }

  JS.Statement _emitLazyFields(JS.Expression objExpr, Iterable<Field> fields) {
    var accessors = <JS.Method>[];
    var savedUri = _currentUri;

    for (var field in fields) {
      _currentUri = field.fileUri;
      var name = field.name.name;
      var access = _emitStaticMemberName(name);
      accessors.add(new JS.Method(access, _emitStaticFieldInitializer(field),
          isGetter: true)
        ..sourceInformation = _hoverComment(
            new JS.PropertyAccess(objExpr, access),
            field.fileOffset,
            name.length));

      // TODO(jmesserly): currently uses a dummy setter to indicate writable.
      if (!field.isFinal && !field.isConst) {
        accessors.add(new JS.Method(access, js.call('function(_) {}') as JS.Fun,
            isSetter: true));
      }
    }
    _currentUri = _currentLibrary.fileUri;

    _currentUri = savedUri;
    return _callHelperStatement('defineLazy(#, { # });', [objExpr, accessors]);
  }

  JS.Fun _emitStaticFieldInitializer(Field field) {
    return new JS.Fun(
        [],
        new JS.Block(_withLetScope(() => [
              new JS.Return(
                  _visitInitializer(field.initializer, field.annotations))
            ])));
  }

  List<JS.Statement> _withLetScope(List<JS.Statement> visitBody()) {
    var savedLetVariables = _letVariables;
    _letVariables = [];

    var body = visitBody();
    var letVars = _initLetVariables();
    if (letVars != null) body.insert(0, letVars);

    _letVariables = savedLetVariables;
    return body;
  }

  JS.ArrowFun _arrowFunctionWithLetScope(JS.Expression visitBody()) {
    var savedLetVariables = _letVariables;
    _letVariables = [];

    var expr = visitBody();
    var letVars = _initLetVariables();

    _letVariables = savedLetVariables;
    return new JS.ArrowFun(
        [], letVars == null ? expr : new JS.Block([letVars, expr.toReturn()]));
  }

  JS.PropertyAccess _emitTopLevelName(NamedNode n, {String suffix: ''}) {
    return _emitJSInterop(n) ?? _emitTopLevelNameNoInterop(n, suffix: suffix);
  }

  /// Like [_emitMemberName], but for declaration sites.
  ///
  /// Unlike call sites, we always have an element available, so we can use it
  /// directly rather than computing the relevant options for [_emitMemberName].
  JS.Expression _declareMemberName(Member m, {bool useExtension}) {
    return _emitMemberName(m.name.name,
        isStatic: m is Field ? m.isStatic : (m as Procedure).isStatic,
        useExtension:
            useExtension ?? _extensionTypes.isNativeClass(m.enclosingClass),
        member: m);
  }

  /// This handles member renaming for private names and operators.
  ///
  /// Private names are generated using ES6 symbols:
  ///
  ///     // At the top of the module:
  ///     let _x = Symbol('_x');
  ///     let _y = Symbol('_y');
  ///     ...
  ///
  ///     class Point {
  ///       Point(x, y) {
  ///         this[_x] = x;
  ///         this[_y] = y;
  ///       }
  ///       get x() { return this[_x]; }
  ///       get y() { return this[_y]; }
  ///     }
  ///
  /// For user-defined operators the following names are allowed:
  ///
  ///     <, >, <=, >=, ==, -, +, /, ~/, *, %, |, ^, &, <<, >>, []=, [], ~
  ///
  /// They generate code like:
  ///
  ///     x['+'](y)
  ///
  /// There are three exceptions: [], []= and unary -.
  /// The indexing operators we use `get` and `set` instead:
  ///
  ///     x.get('hi')
  ///     x.set('hi', 123)
  ///
  /// This follows the same pattern as ECMAScript 6 Map:
  /// <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map>
  ///
  /// Unary minus looks like: `x._negate()`.
  ///
  /// Equality is a bit special, it is generated via the Dart `equals` runtime
  /// helper, that checks for null. The user defined method is called '=='.
  ///
  JS.Expression _emitMemberName(String name,
      {DartType type,
      bool isStatic: false,
      bool useExtension,
      NamedNode member}) {
    // Static members skip the rename steps and may require JS interop renames.
    if (isStatic) {
      return _emitStaticMemberName(name, member);
    }

    // We allow some (illegal in Dart) member names to be used in our private
    // SDK code. These renames need to be included at every declaration,
    // including overrides in subclasses.
    if (member != null) {
      var runtimeName = getJSExportName(member);
      if (runtimeName != null) {
        var parts = runtimeName.split('.');
        if (parts.length < 2) return _propertyName(runtimeName);

        JS.Expression result = new JS.Identifier(parts[0]);
        for (int i = 1; i < parts.length; i++) {
          result = new JS.PropertyAccess(result, _propertyName(parts[i]));
        }
        return result;
      }
    }

    if (name.startsWith('_')) {
      return _emitPrivateNameSymbol(_currentLibrary, name);
    }

    useExtension ??= _isSymbolizedMember(type, name);
    // TODO(vsm): Do not rename members that conflict with standard JS members
    // if we are actually try to access those JS members via interop.
    name = JS.memberNameForDartMember(name);
    if (useExtension) {
      return _getExtensionSymbolInternal(name);
    }
    return _propertyName(name);
  }

  /// This is an internal method used by [_emitMemberName] and the
  /// optimized `dart:_runtime extensionSymbol` builtin to get the symbol
  /// for `dartx.<name>`.
  ///
  /// Do not call this directly; you want [_emitMemberName], which knows how to
  /// handle the many details involved in naming.
  JS.TemporaryId _getExtensionSymbolInternal(String name) {
    return _extensionSymbols.putIfAbsent(
        name,
        () => new JS.TemporaryId(
            '\$${JS.friendlyNameForDartOperator[name] ?? name}'));
  }

  /// Don't symbolize native members that just forward to the underlying
  /// native member.  We limit this to non-renamed members as the receiver
  /// may be a mock type.
  ///
  /// Note, this is an underlying assumption here that, if another native type
  /// subtypes this one, it also forwards this member to its underlying native
  /// one without renaming.
  bool _isSymbolizedMember(DartType type, String name) {
    while (type is TypeParameterType) {
      type = (type as TypeParameterType).bound;
    }
    if (type == null ||
        type == const DynamicType() ||
        type == coreTypes.objectClass) {
      return isObjectMember(name);
    } else if (type is InterfaceType) {
      var c = getImplementationClass(type) ?? type.classNode;
      if (_extensionTypes.isNativeClass(c)) {
        var member = _lookupForwardedMember(c, name);

        // Fields on a native class are implicitly native.
        // Methods/getters/setters are marked external/native.
        if (member is Field || member is Procedure && member.isExternal) {
          var jsName = getAnnotationName(member, isJSName);
          return jsName != null && jsName != name;
        } else {
          // Non-external members must be symbolized.
          return true;
        }
      }
      // If the receiver *may* be a native type (i.e., an interface allowed to
      // be implemented by a native class), conservatively symbolize - we don't
      // know whether it'll be implemented via forwarding.
      // TODO(vsm): Consider CHA here to be less conservative.
      return _extensionTypes.isNativeInterface(c);
    } else if (type is FunctionType) {
      return true;
    }
    return false;
  }

  var _forwardingCache = new HashMap<Class, Map<String, Member>>();

  Member _lookupForwardedMember(Class c, String name) {
    // We only care about public methods.
    if (name.startsWith('_')) return null;

    var map = _forwardingCache.putIfAbsent(c, () => {});

    return map.putIfAbsent(
        name,
        () =>
            hierarchy.getDispatchTarget(c, new Name(name)) ??
            hierarchy.getDispatchTarget(c, new Name(name), setter: true));
  }

  JS.TemporaryId _emitPrivateNameSymbol(Library library, String name) {
    return _privateNames
        .putIfAbsent(library, () => new HashMap())
        .putIfAbsent(name, () {
      var id = new JS.TemporaryId(name);
      _moduleItems.add(
          js.statement('const # = Symbol(#);', [id, js.string(id.name, "'")]));
      return id;
    });
  }

  JS.Expression _emitStaticMemberName(String name, [NamedNode member]) {
    if (member != null) {
      var jsName = _emitJSInteropStaticMemberName(member);
      if (jsName != null) return jsName;
    }

    switch (name) {
      // Reserved for the compiler to do `x as T`.
      case 'as':
      // Reserved for the compiler to do implicit cast `T x = y`.
      case '_check':
      // Reserved for the SDK to compute `Type.toString()`.
      case 'name':
      // Reserved by JS, not a valid static member name.
      case 'prototype':
        name += '_';
        break;
      default:
        // All trailing underscores static names are reserved for the compiler
        // or SDK libraries.
        //
        // If user code uses them, add an extra `_`.
        //
        // This also avoids collision with the renames above, e.g. `static as`
        // and `static as_` will become `as_` and `as__`.
        if (name.endsWith('_')) {
          name += '_';
        }
    }
    return _propertyName(name);
  }

  JS.Expression _emitJSInteropStaticMemberName(NamedNode n) {
    if (!isJSElement(n)) return null;
    var name = getAnnotationName(n, isPublicJSAnnotation);
    if (name != null) {
      if (name.contains('.')) {
        throw new UnsupportedError(
            'static members do not support "." in their names. '
            'See https://github.com/dart-lang/sdk/issues/27926');
      }
    } else {
      name = getTopLevelName(n);
    }
    return js.escapedString(name, "'");
  }

  JS.PropertyAccess _emitTopLevelNameNoInterop(NamedNode n,
      {String suffix: ''}) {
    var name = getJSExportName(n) ?? getTopLevelName(n);
    return new JS.PropertyAccess(
        emitLibraryName(getLibrary(n)), _propertyName(name + suffix));
  }

  String _getJSNameWithoutGlobal(NamedNode n) {
    if (!isJSElement(n)) return null;
    var libraryJSName = getAnnotationName(getLibrary(n), isPublicJSAnnotation);
    var jsName =
        getAnnotationName(n, isPublicJSAnnotation) ?? getTopLevelName(n);
    return libraryJSName != null ? '$libraryJSName.$jsName' : jsName;
  }

  JS.PropertyAccess _emitJSInterop(NamedNode n) {
    var jsName = _getJSNameWithoutGlobal(n);
    if (jsName == null) return null;
    return _emitJSInteropForGlobal(jsName);
  }

  JS.PropertyAccess _emitJSInteropForGlobal(String name) {
    var parts = name.split('.');
    if (parts.isEmpty) parts = [''];
    JS.PropertyAccess access;
    for (var part in parts) {
      access = new JS.PropertyAccess(
          access ?? _callHelper('global'), js.escapedString(part, "'"));
    }
    return access;
  }

  void _emitLibraryProcedures(Library library) {
    var procedures = library.procedures
        .where((p) => !p.isExternal && !p.isAbstract)
        .toList();
    _moduleItems.addAll(procedures
        .where((p) => !p.isAccessor)
        .map(_emitLibraryFunction)
        .toList());
    _emitLibraryAccessors(procedures.where((p) => p.isAccessor).toList());
  }

  void _emitLibraryAccessors(Iterable<Procedure> accessors) {
    if (accessors.isEmpty) return;
    _moduleItems.add(_callHelperStatement('copyProperties(#, { # });', [
      emitLibraryName(_currentLibrary),
      accessors.map(_emitLibraryAccessor).toList()
    ]));
  }

  JS.Method _emitLibraryAccessor(Procedure node) {
    var savedUri = _currentUri;
    _currentUri = node.fileUri;

    var name = node.name.name;
    var result = new JS.Method(
        _propertyName(name), _emitFunction(node.function, node.name.name),
        isGetter: node.isGetter, isSetter: node.isSetter)
      ..sourceInformation = _nodeEnd(node.fileEndOffset);

    _currentUri = savedUri;
    return result;
  }

  JS.Statement _emitLibraryFunction(Procedure p) {
    var savedUri = _currentUri;
    _currentUri = p.fileUri;

    var body = <JS.Statement>[];
    var fn = _emitFunction(p.function, p.name.name)
      ..sourceInformation = _nodeEnd(p.fileEndOffset);

    if (_currentLibrary.importUri.scheme == 'dart' &&
        _isInlineJSFunction(p.function.body)) {
      fn = JS.simplifyPassThroughArrowFunCallBody(fn);
    }

    var nameExpr = _emitTopLevelName(p);
    body.add(js.statement('# = #', [nameExpr, fn]));
    if (!isSdkInternalRuntime(_currentLibrary)) {
      body.add(
          _emitFunctionTagged(nameExpr, p.function.functionType, topLevel: true)
              .toStatement());
    }

    _currentUri = savedUri;
    return JS.Statement.from(body);
  }

  JS.Expression _emitFunctionTagged(JS.Expression fn, FunctionType type,
      {bool topLevel: false}) {
    var lazy = topLevel && !_typeIsLoaded(type);
    var typeRep = visitFunctionType(type);
    return _callHelper(lazy ? 'lazyFn(#, () => #)' : 'fn(#, #)', [fn, typeRep]);
  }

  bool _typeIsLoaded(DartType type) {
    if (type is InterfaceType) {
      return !_pendingClasses.contains(type.classNode) &&
          type.typeArguments.every(_typeIsLoaded);
    }
    if (type is FunctionType) {
      return (_typeIsLoaded(type.returnType) &&
          type.positionalParameters.every(_typeIsLoaded) &&
          type.namedParameters.every((n) => _typeIsLoaded(n.type)));
    }
    if (type is TypedefType) {
      return type.typeArguments.every(_typeIsLoaded);
    }
    return true;
  }

  /// Emits a Dart [type] into code.
  JS.Expression _emitType(DartType type) => type.accept(this) as JS.Expression;

  JS.Expression _emitInvalidNode(Node node, [String message = '']) {
    if (message.isNotEmpty) message += ' ';
    return _callHelper('throwUnimplementedError(#)',
        [js.escapedString('node <${node.runtimeType}> $message`$node`')]);
  }

  JS.Expression _nameType(DartType type, JS.Expression typeRep) =>
      _currentFunction != null ? _typeTable.nameType(type, typeRep) : typeRep;

  @override
  defaultDartType(type) => _emitInvalidNode(type);

  @override
  visitInvalidType(type) => defaultDartType(type);

  @override
  visitDynamicType(type) => _callHelper('dynamic');

  @override
  visitVoidType(type) => _callHelper('void');

  @override
  visitBottomType(type) => _callHelper('bottom');

  @override
  visitInterfaceType(type, {bool lowerGeneric: false}) {
    var c = type.classNode;
    _declareBeforeUse(c);

    // Type parameters don't matter as JS interop types cannot be reified.
    // We have to use lazy JS types because until we have proper module
    // loading for JS libraries bundled with Dart libraries, we will sometimes
    // need to load Dart libraries before the corresponding JS libraries are
    // actually loaded.
    // Given a JS type such as:
    //     @JS('google.maps.Location')
    //     class Location { ... }
    // We can't emit a reference to MyType because the JS library that defines
    // it may be loaded after our code. So for now, we use a special lazy type
    // object to represent MyType.
    // Anonymous JS types do not have a corresponding concrete JS type so we
    // have to use a helper to define them.
    if (isJSAnonymousType(c)) {
      return _callHelper(
          'anonymousJSType(#)', js.escapedString(getLocalClassName(c)));
    }
    var jsName = _getJSNameWithoutGlobal(c);
    if (jsName != null) {
      return _callHelper('lazyJSType(() => #, #)',
          [_emitJSInteropForGlobal(jsName), js.escapedString(jsName)]);
    }

    var args = type.typeArguments;
    Iterable<JS.Expression> jsArgs = null;
    if (args.any((a) => a != const DynamicType())) {
      jsArgs = args.map(_emitType);
    } else if (lowerGeneric) {
      jsArgs = [];
    }
    if (jsArgs != null) {
      return _nameType(type, _emitGenericClassType(type, jsArgs));
    }

    return _emitTopLevelNameNoInterop(type.classNode);
  }

  JS.Expression _emitGenericClassType(
      InterfaceType t, Iterable<JS.Expression> typeArgs) {
    var genericName = _emitTopLevelNameNoInterop(t.classNode, suffix: '\$');
    return js.call('#(#)', [genericName, typeArgs]);
  }

  @override
  visitVectorType(type) => defaultDartType(type);

  @override
  visitFunctionType(type, {bool lowerTypedef: false, FunctionNode function}) {
    var requiredTypes =
        type.positionalParameters.take(type.requiredParameterCount).toList();
    var requiredParams = function?.positionalParameters
        ?.take(type.requiredParameterCount)
        ?.toList();
    var optionalTypes =
        type.positionalParameters.skip(type.requiredParameterCount).toList();
    var optionalParams = function?.positionalParameters
        ?.skip(type.requiredParameterCount)
        ?.toList();

    var namedTypes = type.namedParameters;
    var rt = _emitType(type.returnType);
    var ra = _emitTypeNames(requiredTypes, requiredParams);

    List<JS.Expression> typeParts;
    if (namedTypes.isNotEmpty) {
      assert(optionalTypes.isEmpty);
      // TODO(vsm): Pass in annotations here as well.
      var na = _emitTypeProperties(namedTypes);
      typeParts = [rt, ra, na];
    } else if (optionalTypes.isNotEmpty) {
      assert(namedTypes.isEmpty);
      var oa = _emitTypeNames(optionalTypes, optionalParams);
      typeParts = [rt, ra, oa];
    } else {
      typeParts = [rt, ra];
    }

    var typeFormals = type.typeParameters;
    String helperCall;
    if (typeFormals.isNotEmpty) {
      var tf = _emitTypeFormals(typeFormals);

      addTypeFormalsAsParameters(List<JS.Expression> elements) {
        var names = _typeTable.discharge(typeFormals);
        return names.isEmpty
            ? js.call('(#) => [#]', [tf, elements])
            : js.call('(#) => {#; return [#];}', [tf, names, elements]);
      }

      typeParts = [addTypeFormalsAsParameters(typeParts)];

      helperCall = 'gFnType(#)';
      // If any explicit bounds were passed, emit them.
      if (typeFormals.any((t) => t.bound != null)) {
        var bounds = typeFormals.map((t) => _emitType(t.bound)).toList();
        typeParts.add(addTypeFormalsAsParameters(bounds));
      }
    } else {
      helperCall = 'fnType(#)';
    }
    return _nameType(type, _callHelper(helperCall, [typeParts]));
  }

  JS.Expression _emitAnnotatedFunctionType(
      FunctionType type, List<Expression> metadata,
      {FunctionNode function, bool nameType: true, bool definite: false}) {
    var result = visitFunctionType(type, function: function);
    return _emitAnnotatedResult(result, metadata);
  }

  /// Emits an expression that lets you access statics on a [type] from code.
  JS.Expression _emitConstructorAccess(InterfaceType type) {
    return _emitJSInterop(type.classNode) ?? _emitType(type);
  }

  JS.Expression _emitConstructorName(InterfaceType type, Member c) {
    return _emitJSInterop(type.classNode) ??
        new JS.PropertyAccess(
            _emitConstructorAccess(type), _constructorName(c.name.name));
  }

  /// Emits an expression that lets you access statics on an [element] from code.
  JS.Expression _emitStaticAccess(Class c) {
    _declareBeforeUse(c);
    return _emitTopLevelName(c);
  }

  // Wrap a result - usually a type - with its metadata.  The runtime is
  // responsible for unpacking this.
  JS.Expression _emitAnnotatedResult(
      JS.Expression result, List<Expression> metadata) {
    if (emitMetadata && metadata != null && metadata.isNotEmpty) {
      result = new JS.ArrayInitializer(
          [result]..addAll(metadata.map(_instantiateAnnotation)));
    }
    return result;
  }

  JS.ObjectInitializer _emitTypeProperties(Iterable<NamedType> types) {
    return new JS.ObjectInitializer(types
        .map((t) => new JS.Property(_propertyName(t.name), _emitType(t.type)))
        .toList());
  }

  JS.ArrayInitializer _emitTypeNames(
      List<DartType> types, List<VariableDeclaration> parameters) {
    var result = <JS.Expression>[];
    for (int i = 0; i < types.length; ++i) {
      var metadata = parameters != null ? parameters[i].annotations : null;
      result.add(_emitAnnotatedResult(_emitType(types[i]), metadata));
    }
    return new JS.ArrayInitializer(result);
  }

  @override
  visitTypeParameterType(type) => _emitTypeParameter(type.parameter);

  JS.Identifier _emitTypeParameter(TypeParameter t) {
    _typeParamInConst?.add(t);
    return new JS.Identifier(getTypeParameterName(t));
  }

  @override
  visitTypedefType(type, {bool lowerGeneric: false}) {
    var args = type.typeArguments;
    List<JS.Expression> jsArgs = null;
    if (args.any((a) => a != const DynamicType())) {
      jsArgs = args.map(_emitType).toList();
    } else if (lowerGeneric) {
      jsArgs = [];
    }
    if (jsArgs != null) {
      var genericName =
          _emitTopLevelNameNoInterop(type.typedefNode, suffix: '\$');
      return _nameType(type, new JS.Call(genericName, jsArgs));
    }

    return _emitTopLevelNameNoInterop(type.typedefNode);
  }

  JS.Fun _emitFunction(FunctionNode f, String name) {
    // normal function (sync), vs (sync*, async, async*)
    var isSync = f.asyncMarker == AsyncMarker.Sync;
    var formals = _emitFormalParameters(f);
    var typeFormals = _emitTypeFormals(f.typeParameters);
    formals.insertAll(0, typeFormals);

    JS.Block code = isSync
        ? _emitFunctionBody(f)
        : new JS.Block([
            _emitGeneratorFunction(f, name).toReturn()
              ..sourceInformation = _nodeStart(f)
          ]);

    if (name != null && formals.isNotEmpty) {
      if (name == '[]=') {
        // []= methods need to return the value. We could also address this at
        // call sites, but it's cleaner to instead transform the operator method.
        code = JS.alwaysReturnLastParameter(code, formals.last);
      } else if (name == '==' && _currentLibrary.importUri.scheme != 'dart') {
        // In Dart `operator ==` methods are not called with a null argument.
        // This is handled before calling them. For performance reasons, we push
        // this check inside the method, to simplify our `equals` helper.
        //
        // TODO(jmesserly): in most cases this check is not necessary, because
        // the Dart code already handles it (typically by an `is` check).
        // Eliminate it when possible.
        code = new JS.Block([
          js.statement('if (# == null) return false;', [formals.first]),
          code
        ]);
      }
    }

    return new JS.Fun(formals, code);
  }

  // TODO(jmesserly): rename _emitParameters
  List<JS.Parameter> _emitFormalParameters(FunctionNode f) {
    var result = f.positionalParameters.map(_emitVariableDef).toList();
    if (f.namedParameters.isNotEmpty) result.add(namedArgumentTemp);
    return result;
  }

  void _emitVirtualFieldSymbols(Class c, List<JS.Statement> body) {
    _classProperties.virtualFields.forEach((field, virtualField) {
      body.add(js.statement('const # = Symbol(#);', [
        virtualField,
        js.string('${getLocalClassName(c)}.${field.name.name}')
      ]));
    });
  }

  List<JS.Identifier> _emitTypeFormals(List<TypeParameter> typeFormals) {
    return typeFormals
        .map((t) => new JS.Identifier(getTypeParameterName(t)))
        .toList();
  }