pkg/dev_compiler/lib/src/compiler/code_generator.dart

// Copyright (c) 2015, 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' show HashMap, HashSet;
import 'dart:math' show min, max;

import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
import 'package:analyzer/dart/ast/ast.dart';
import 'package:analyzer/dart/ast/token.dart' show Token, TokenType;
import 'package:analyzer/dart/element/element.dart';
import 'package:analyzer/dart/element/type.dart';
import 'package:analyzer/src/dart/ast/token.dart' show StringToken;
import 'package:analyzer/src/dart/element/element.dart'
    show LocalVariableElementImpl;
import 'package:analyzer/src/dart/element/type.dart' show DynamicTypeImpl;
import 'package:analyzer/src/dart/sdk/sdk.dart';
import 'package:analyzer/src/generated/engine.dart' show AnalysisContext;
import 'package:analyzer/src/generated/resolver.dart'
    show TypeProvider, NamespaceBuilder;
import 'package:analyzer/src/generated/type_system.dart'
    show StrongTypeSystemImpl;
import 'package:analyzer/src/summary/idl.dart' show UnlinkedUnit;
import 'package:analyzer/src/summary/link.dart' as summary_link;
import 'package:analyzer/src/summary/package_bundle_reader.dart';
import 'package:analyzer/src/summary/summarize_ast.dart'
    show serializeAstUnlinked;
import 'package:analyzer/src/summary/summarize_elements.dart'
    show PackageBundleAssembler;
import 'package:analyzer/src/summary/summary_sdk.dart';
import 'package:analyzer/src/task/strong/ast_properties.dart'
    show isDynamicInvoke, setIsDynamicInvoke, getImplicitAssignmentCast;
import 'package:path/path.dart' show separator;

import '../closure/closure_annotator.dart' show ClosureAnnotator;
import '../js_ast/js_ast.dart' as JS;
import '../js_ast/js_ast.dart' show js;
import 'ast_builder.dart' show AstBuilder;
import 'compiler.dart' show BuildUnit, CompilerOptions, JSModuleFile;
import 'element_helpers.dart';
import 'element_loader.dart' show ElementLoader;
import 'extension_types.dart' show ExtensionTypeSet;
import 'js_field_storage.dart' show checkForPropertyOverride, getSuperclasses;
import 'js_interop.dart';
import 'js_metalet.dart' as JS;
import 'js_names.dart' as JS;
import 'js_typeref_codegen.dart' show JsTypeRefCodegen;
import 'module_builder.dart' show pathToJSIdentifier;
import 'nullable_type_inference.dart' show NullableTypeInference;
import 'reify_coercions.dart' show CoercionReifier;
import 'side_effect_analysis.dart' show ConstFieldVisitor, isStateless;
import 'type_utilities.dart';

class CodeGenerator extends GeneralizingAstVisitor
    with ClosureAnnotator, JsTypeRefCodegen, NullableTypeInference {
  final AnalysisContext context;
  final SummaryDataStore summaryData;

  final CompilerOptions options;
  final rules = new StrongTypeSystemImpl();

  /// 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<LibraryElement, JS.Identifier>();

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

  /// The list of output module items, in the order they need to be emitted in.
  final _moduleItems = <JS.ModuleItem>[];

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

  /// The global extension type table.
  final ExtensionTypeSet _extensionTypes;

  /// The variable for the target of the current `..` cascade expression.
  ///
  /// Usually a [SimpleIdentifier], but it can also be other expressions
  /// that are safe to evaluate multiple times, such as `this`.
  Expression _cascadeTarget;

  /// The variable for the current catch clause
  SimpleIdentifier _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<LibraryElement, HashMap<String, JS.TemporaryId>>();
  final _initializingFormalTemps =
      new HashMap<ParameterElement, JS.TemporaryId>();

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

  final _hasDeferredSupertype = new HashSet<ClassElement>();

  final _eagerTopLevelFields = new HashSet<Element>.identity();

  /// The  type provider from the current Analysis [context].
  final TypeProvider types;

  final LibraryElement dartCoreLibrary;
  final LibraryElement dartJSLibrary;

  /// The dart:async `StreamIterator<>` type.
  final InterfaceType _asyncStreamIterator;

  /// The dart:_interceptors JSArray element.
  final ClassElement _jsArray;

  final ClassElement boolClass;
  final ClassElement intClass;
  final ClassElement interceptorClass;
  final ClassElement nullClass;
  final ClassElement numClass;
  final ClassElement objectClass;
  final ClassElement stringClass;
  final ClassElement symbolClass;

  ConstFieldVisitor _constants;

  /// The current function body being compiled.
  FunctionBody _currentFunction;

  /// Helper class for emitting elements in the proper order to allow
  /// JS to load the module.
  ElementLoader _loader;

  BuildUnit _buildUnit;

  String _libraryRoot;

  bool _superAllowed = true;

  List<JS.TemporaryId> _superHelperSymbols = <JS.TemporaryId>[];
  List<JS.Method> _superHelpers = <JS.Method>[];

  List<TypeParameterType> _typeParamInConst = null;

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

  CodeGenerator(
      AnalysisContext c, this.summaryData, this.options, this._extensionTypes)
      : context = c,
        types = c.typeProvider,
        _asyncStreamIterator =
            _getLibrary(c, 'dart:async').getType('StreamIterator').type,
        _jsArray = _getLibrary(c, 'dart:_interceptors').getType('JSArray'),
        interceptorClass =
            _getLibrary(c, 'dart:_interceptors').getType('Interceptor'),
        dartCoreLibrary = _getLibrary(c, 'dart:core'),
        boolClass = _getLibrary(c, 'dart:core').getType('bool'),
        intClass = _getLibrary(c, 'dart:core').getType('int'),
        numClass = _getLibrary(c, 'dart:core').getType('num'),
        nullClass = _getLibrary(c, 'dart:core').getType('Null'),
        objectClass = _getLibrary(c, 'dart:core').getType('Object'),
        stringClass = _getLibrary(c, 'dart:core').getType('String'),
        symbolClass = _getLibrary(c, 'dart:_internal').getType('Symbol'),
        dartJSLibrary = _getLibrary(c, 'dart:js');

  LibraryElement get currentLibrary => _loader.currentElement.library;

  /// The main entry point to JavaScript code generation.
  ///
  /// Takes the metadata for the build unit, as well as resolved trees and
  /// errors, and computes the output module code and optionally the source map.
  JSModuleFile compile(BuildUnit unit, List<CompilationUnit> compilationUnits,
      List<String> errors) {
    _buildUnit = unit;
    _libraryRoot = _buildUnit.libraryRoot;
    if (!_libraryRoot.endsWith(separator)) {
      _libraryRoot = '$_libraryRoot${separator}';
    }

    var module = _emitModule(compilationUnits);
    var dartApiSummary = _summarizeModule(compilationUnits);

    return new JSModuleFile(unit.name, errors, options, module, dartApiSummary);
  }

  List<int> _summarizeModule(List<CompilationUnit> units) {
    if (!options.summarizeApi) return null;

    if (!units.any((u) => u.element.librarySource.isInSystemLibrary)) {
      var sdk = context.sourceFactory.dartSdk;
      summaryData.addBundle(
          null,
          sdk is SummaryBasedDartSdk
              ? sdk.bundle
              : (sdk as FolderBasedDartSdk).getSummarySdkBundle(true));
    }

    var assembler = new PackageBundleAssembler();
    assembler.recordDependencies(summaryData);

    var uriToUnit = new Map<String, UnlinkedUnit>.fromIterable(units,
        key: (u) => u.element.source.uri.toString(), value: (unit) {
      var unlinked = serializeAstUnlinked(unit);
      assembler.addUnlinkedUnit(unit.element.source, unlinked);
      return unlinked;
    });

    summary_link
        .link(
            uriToUnit.keys.toSet(),
            (uri) => summaryData.linkedMap[uri],
            (uri) => summaryData.unlinkedMap[uri] ?? uriToUnit[uri],
            context.declaredVariables.get,
            true)
        .forEach(assembler.addLinkedLibrary);

    var bundle = assembler.assemble();
    // Preserve only API-level information in the summary.
    bundle.flushInformative();
    return bundle.toBuffer();
  }

  JS.Program _emitModule(List<CompilationUnit> compilationUnits) {
    if (_moduleItems.isNotEmpty) {
      throw new StateError('Can only call emitModule once.');
    }

    // Transform the AST to make coercions explicit.
    compilationUnits = CoercionReifier.reify(compilationUnits);

    if (compilationUnits.any((u) => _isDartRuntime(u.element.library))) {
      // 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');
    } 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>[];
    for (var unit in compilationUnits) {
      var library = unit.element.library;
      if (unit.element != library.definingCompilationUnit) continue;

      var libraryTemp = _isDartRuntime(library)
          ? _runtimeModule
          : new JS.TemporaryId(jsLibraryName(_libraryRoot, library));
      _libraries[library] = libraryTemp;
      items.add(new JS.ExportDeclaration(
          js.call('const # = Object.create(null)', [libraryTemp])));

      // dart:_runtime has a magic module that holds extension method symbols.
      // TODO(jmesserly): find a cleaner design for this.
      if (_isDartRuntime(library)) {
        items.add(new JS.ExportDeclaration(js
            .call('const # = Object.create(null)', [_extensionSymbolsModule])));
      }
    }

    // Collect all Element -> Node mappings, in case we need to forward declare
    // any nodes.
    var nodes = new HashMap<Element, AstNode>.identity();
    var sdkBootstrappingFns = new List<FunctionElement>();
    for (var unit in compilationUnits) {
      if (_isDartRuntime(unit.element.library)) {
        sdkBootstrappingFns.addAll(unit.element.functions);
      }
      _collectElements(unit, nodes);
    }
    _loader = new ElementLoader(nodes);
    if (compilationUnits.isNotEmpty) {
      _constants = new ConstFieldVisitor(context,
          dummySource: compilationUnits.first.element.source);
    }

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

    // Emit SDK bootstrapping functions first, if any.
    sdkBootstrappingFns.forEach(_emitDeclaration);

    // Visit each compilation unit and emit its code.
    //
    // NOTE: declarations 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.
    compilationUnits.forEach(_finishDeclarationsInUnit);

    // Declare imports
    _finishImports(items);

    // 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: _buildUnit.name);
  }

  List<String> _getJSName(Element e) {
    if (e.library == null ||
        findAnnotation(e.library, isPublicJSAnnotation) == null) {
      return null;
    }

    var libraryJSName = getAnnotationName(e.library, isPublicJSAnnotation);
    var libraryPrefix = <String>[];
    if (libraryJSName != null && libraryJSName.isNotEmpty) {
      libraryPrefix.addAll(libraryJSName.split('.'));
    }

    String elementJSName;
    if (findAnnotation(e, isPublicJSAnnotation) != null) {
      elementJSName = getAnnotationName(e, isPublicJSAnnotation) ?? '';
    }

    if (e is TopLevelVariableElement) {
      elementJSName = _jsInteropStaticMemberName(e);
    }
    if (elementJSName == null) return null;

    var elementJSParts = <String>[];
    if (elementJSName.isNotEmpty) {
      elementJSParts.addAll(elementJSName.split('.'));
    } else {
      elementJSParts.add(e.name);
    }

    return libraryPrefix..addAll(elementJSParts);
  }

  JS.Expression _emitJSInterop(Element e) {
    var jsName = _getJSName(e);
    if (jsName == null) return null;
    var fullName = ['global']..addAll(jsName);
    JS.Expression access = _runtimeModule;
    for (var part in fullName) {
      access = new JS.PropertyAccess(access, js.string(part));
    }
    return access;
  }

  String _jsInteropStaticMemberName(Element e, {String name}) {
    if (e == null ||
        e.library == null ||
        findAnnotation(e.library, isPublicJSAnnotation) == null) {
      return null;
    }
    if (e is PropertyInducingElement) {
      // Assume properties have consistent JS names for getters and setters.
      return _jsInteropStaticMemberName(e.getter, name: e.name) ??
          _jsInteropStaticMemberName(e.setter, name: e.name);
    }
    if (e is ExecutableElement &&
        e.isExternal &&
        findAnnotation(e, isPublicJSAnnotation) != null) {
      return getAnnotationName(e, isPublicJSAnnotation) ?? name ?? e.name;
    }
    return null;
  }

  JS.Expression _emitJSInteropStaticMemberName(Element e) {
    var name = _jsInteropStaticMemberName(e);
    if (name == null) return null;
    // We do not support statics names with JS annotations containing dots.
    // See https://github.com/dart-lang/sdk/issues/27926
    if (name.contains('.')) {
      throw new UnimplementedError(
          'We do not support JS annotations containing dots on static members. '
          'See https://github.com/dart-lang/sdk/issues/27926');
    }
    return js.string(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 {
        result.add(item);
      }
    }
  }

  String _libraryToModule(LibraryElement library) {
    assert(!_libraries.containsKey(library));
    var source = library.source;
    // TODO(jmesserly): we need to split out HTML.
    if (source.uri.scheme == 'dart') {
      return 'dart_sdk';
    }
    var moduleName = _buildUnit.libraryToModule(source);
    if (moduleName == null) {
      throw new StateError('Could not find module containing "$library".');
    }
    return moduleName;
  }

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

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

    String coreModuleName;
    if (!_libraries.containsKey(dartCoreLibrary)) {
      coreModuleName = _libraryToModule(dartCoreLibrary);
    }
    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, "'")));
    });
  }

  /// Collect toplevel elements and nodes we need to emit, and returns
  /// an ordered map of these.
  static void _collectElements(
      CompilationUnit unit, Map<Element, AstNode> map) {
    for (var declaration in unit.declarations) {
      if (declaration is TopLevelVariableDeclaration) {
        for (var field in declaration.variables.variables) {
          map[field.element] = field;
        }
      } else {
        map[declaration.element] = declaration;
      }
    }
  }

  /// Called to emit all top-level 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 _emitDeclaration(Element e) {
    var item = _loader.emitDeclaration(e, (AstNode node) {
      // TODO(jmesserly): this is not really the right place for this.
      // Ideally we do this per function body.
      //
      // We'll need to be consistent about when we're generating functions, and
      // only run this on the outermost function, and not any closures.
      inferNullableTypes(node);
      return _visit(node);
    });

    if (item != null) _moduleItems.add(item);
  }

  void _declareBeforeUse(Element e) {
    _loader.declareBeforeUse(e, _emitDeclaration);
  }

  void _finishDeclarationsInUnit(CompilationUnit unit) {
    // NOTE: this method isn't the right place to initialize
    // per-compilation-unit state. Declarations can be visited out of order,
    // this is only to catch things that haven't been emitted yet.
    //
    // See _emitDeclaration.
    for (var declaration in unit.declarations) {
      var element = declaration.element;
      if (element != null) {
        _emitDeclaration(element);
      } else {
        declaration.accept(this);
      }
    }
    for (var directive in unit.directives) {
      directive.accept(this);
    }
  }

  @override
  void visitLibraryDirective(LibraryDirective node) {}

  @override
  void visitImportDirective(ImportDirective node) {
    // We don't handle imports here.
    //
    // Instead, we collect imports whenever we need to generate a reference
    // to another library. This has the effect of collecting the actually used
    // imports.
    //
    // TODO(jmesserly): if this is a prefixed import, consider adding the prefix
    // as an alias?
  }

  @override
  void visitPartDirective(PartDirective node) {}

  @override
  void visitPartOfDirective(PartOfDirective node) {}

  @override
  void visitExportDirective(ExportDirective node) {
    ExportElement element = node.element;
    var currentLibrary = element.library;

    var currentNames = currentLibrary.publicNamespace.definedNames;
    var exportedNames =
        new NamespaceBuilder().createExportNamespaceForDirective(element);

    var libraryName = emitLibraryName(currentLibrary);

    // TODO(jmesserly): we could collect all of the names for bulk re-export,
    // but this is easier to implement for now.
    void emitExport(Element export, {String suffix: ''}) {
      var name = _emitTopLevelName(export, suffix: suffix);

      if (export is TypeDefiningElement ||
          export is FunctionElement ||
          _eagerTopLevelFields.contains(export)) {
        // classes, typedefs, functions, and eager init fields can be assigned
        // directly.
        // TODO(jmesserly): we don't know about eager init fields from other
        // modules we import, so we will never go down this code path for them.
        _moduleItems
            .add(js.statement('#.# = #;', [libraryName, name.selector, name]));
      }
    }

    // We only need to export main as it is the only method party 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 export = exportedNames.get('main');

    if (export == null) return;
    if (export is PropertyAccessorElement) {
      export = (export as PropertyAccessorElement).variable;
    }

    // Don't allow redefining names from this library.
    if (currentNames.containsKey(export.name)) return;

    if (export.isSynthetic && export is PropertyInducingElement) {
      _emitDeclaration(export.getter);
      _emitDeclaration(export.setter);
    } else {
      _emitDeclaration(export);
    }
    emitExport(export);
  }

  @override
  visitAsExpression(AsExpression node) {
    Expression fromExpr = node.expression;
    var from = getStaticType(fromExpr);
    var to = node.type.type;

    JS.Expression jsFrom = _visit(fromExpr);
    if (_inWhitelistCode(node)) return jsFrom;

    // Skip the cast if it's not needed.
    if (rules.isSubtypeOf(from, to)) return jsFrom;

    // All Dart number types map to a JS double.
    if (_isNumberInJS(from) && _isNumberInJS(to)) {
      // Make sure to check when converting to int.
      if (from != types.intType && to == types.intType) {
        // TODO(jmesserly): fuse this with notNull check.
        return _callHelper('asInt(#)', jsFrom);
      }

      // A no-op in JavaScript.
      return jsFrom;
    }

    var type = _emitType(to,
        nameType: options.nameTypeTests || options.hoistTypeTests,
        hoistType: options.hoistTypeTests);
    if (CoercionReifier.isImplicitCast(node)) {
      return js.call('#._check(#)', [type, jsFrom]);
    } else {
      return js.call('#.as(#)', [type, jsFrom]);
    }
  }

  @override
  visitIsExpression(IsExpression node) {
    // Generate `is` as `dart.is` or `typeof` depending on the RHS type.
    JS.Expression result;
    var type = node.type.type;
    var lhs = _visit(node.expression);
    var typeofName = _jsTypeofName(type);
    if (typeofName != null) {
      result = js.call('typeof # == #', [lhs, js.string(typeofName, "'")]);
    } else {
      // Always go through a runtime helper, because implicit interfaces.

      var castType = _emitType(type,
          nameType: options.nameTypeTests || options.hoistTypeTests,
          hoistType: options.hoistTypeTests);

      result = js.call('#.is(#)', [castType, lhs]);
    }

    if (node.notOperator != null) {
      return js.call('!#', result);
    }
    return result;
  }

  String _jsTypeofName(DartType t) {
    if (_isNumberInJS(t)) return 'number';
    if (t == types.stringType) return 'string';
    if (t == types.boolType) return 'boolean';
    return null;
  }

  @override
  visitFunctionTypeAlias(FunctionTypeAlias node) {
    FunctionTypeAliasElement element = node.element;

    JS.Expression body = annotate(
        _callHelper('typedef(#, () => #)', [
          js.string(element.name, "'"),
          _emitType(element.type, nameType: false, lowerTypedef: true)
        ]),
        node,
        element);

    var typeFormals = element.typeParameters;
    if (typeFormals.isNotEmpty) {
      return _defineClassTypeArguments(element, typeFormals,
          js.statement('const # = #;', [element.name, body]));
    } else {
      return js.statement('# = #;', [_emitTopLevelName(element), body]);
    }
  }

  @override
  JS.Expression visitTypeName(TypeName node) {
    if (node.type == null) {
      // TODO(jmesserly): if the type fails to resolve, should we generate code
      // that throws instead?
      assert(options.unsafeForceCompile || options.replCompile);
      return _callHelper('dynamic');
    }
    return _emitType(node.type);
  }

  @override
  JS.Statement visitClassTypeAlias(ClassTypeAlias node) {
    ClassElement element = node.element;

    // Forward all generative constructors from the base class.
    var methods = <JS.Method>[];

    var supertype = element.supertype;
    if (!supertype.isObject) {
      for (var ctor in element.constructors) {
        var parentCtor = supertype.lookUpConstructor(ctor.name, ctor.library);
        // TODO(jmesserly): this avoids spread args for perf. Revisit.
        var jsParams = <JS.Identifier>[];
        for (var p in ctor.parameters) {
          if (p.parameterKind != ParameterKind.NAMED) {
            jsParams.add(new JS.Identifier(p.name));
          } else {
            jsParams.add(new JS.TemporaryId('namedArgs'));
            break;
          }
        }
        var fun = js.call('function(#) { super.#(#); }',
            [jsParams, _constructorName(parentCtor), jsParams]) as JS.Fun;
        methods.add(new JS.Method(_constructorName(ctor), fun));
      }
    }

    var classExpr = _emitClassExpression(element, methods);

    var typeFormals = element.typeParameters;
    if (typeFormals.isNotEmpty) {
      return _defineClassTypeArguments(
          element, typeFormals, new JS.ClassDeclaration(classExpr));
    } else {
      return js.statement('# = #;', [_emitTopLevelName(element), classExpr]);
    }
  }

  JS.Statement _emitJsType(Element e) {
    var jsTypeName = getAnnotationName(e, isJSAnnotation);
    if (jsTypeName == null || jsTypeName == e.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(e), jsTypeName]);
  }

  @override
  JS.Statement visitClassDeclaration(ClassDeclaration node) {
    var classElem = node.element;

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

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

    var ctors = <ConstructorDeclaration>[];
    var fields = <FieldDeclaration>[];
    var staticFields = <FieldDeclaration>[];
    var methods = <MethodDeclaration>[];

    // True if a "call" method or getter exists.
    bool isCallable = false;
    for (var member in node.members) {
      if (member is ConstructorDeclaration) {
        ctors.add(member);
      } else if (member is FieldDeclaration) {
        (member.isStatic ? staticFields : fields).add(member);
      } else if (member is MethodDeclaration) {
        methods.add(member);
        if (member.name.name == 'call' && !member.isSetter) {
          //
          // Make sure "call" has a statically known function type:
          //
          // - if it's a method, then it does because all methods do,
          // - if it's a getter, check the return type.
          //
          // Other cases like a getter returning dynamic/Object/Function will be
          // handled at runtime by the dynamic call mechanism. So we only
          // concern ourselves with statically known function types.
          //
          // For the same reason, we can ignore "noSuchMethod".
          // call-implemented-by-nSM will be dispatched by dcall at runtime.
          //
          isCallable = !member.isGetter || member.returnType is FunctionType;
        }
      }
    }

    JS.Expression className;
    if (classElem.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(classElem.name);
    } else {
      className = _emitTopLevelName(classElem);
    }

    var allFields = fields.toList()..addAll(staticFields);
    var superclasses = getSuperclasses(classElem);
    var virtualFields = <FieldElement, JS.TemporaryId>{};
    var virtualFieldSymbols = <JS.Statement>[];
    var staticFieldOverrides = new HashSet<FieldElement>();
    var extensions = _extensionsToImplement(classElem);
    _registerPropertyOverrides(classElem, className, superclasses, allFields,
        virtualFields, virtualFieldSymbols, staticFieldOverrides, extensions);

    var classExpr = _emitClassExpression(classElem,
        _emitClassMethods(node, ctors, fields, superclasses, virtualFields),
        fields: allFields);

    var body = <JS.Statement>[];
    _initExtensionSymbols(classElem, methods, fields, body);
    _emitSuperHelperSymbols(_superHelperSymbols, body);

    // Emit the class, e.g. `core.Object = class Object { ... }`
    _defineClass(classElem, className, classExpr, isCallable, body);

    // Emit things that come after the ES6 `class ... { ... }`.
    var jsPeerNames = _getJSPeerNames(classElem);
    JS.Statement deferredBaseClass =
        _setBaseClass(classElem, className, jsPeerNames, body);

    _emitClassTypeTests(classElem, className, body);

    _defineNamedConstructors(ctors, body, className, isCallable);
    body.addAll(virtualFieldSymbols);
    _emitClassSignature(
        methods, allFields, classElem, ctors, extensions, className, body);
    _defineExtensionMembers(extensions, className, body);
    _emitClassMetadata(node.metadata, className, body);

    JS.Statement classDef = _statement(body);

    var typeFormals = classElem.typeParameters;
    if (typeFormals.isNotEmpty) {
      classDef = _defineClassTypeArguments(
          classElem, typeFormals, classDef, className, deferredBaseClass);
    }

    body = <JS.Statement>[classDef];
    _emitStaticFields(staticFields, staticFieldOverrides, classElem, body);
    for (var peer in jsPeerNames) {
      _registerExtensionType(classElem, peer, body);
    }
    return _statement(body);
  }

  /// Emits code to support a class with a "call" method and an unnamed
  /// constructor.
  ///
  /// This ensures instances created by the unnamed constructor are functions.
  /// Named constructors are handled elsewhere, see [_defineNamedConstructors].
  JS.Expression _emitCallableClass(
      JS.ClassExpression classExpr, ConstructorElement unnamedCtor) {
    var ctor = new JS.NamedFunction(
        classExpr.name, _emitCallableClassConstructor(unnamedCtor));

    // Name the constructor function the same as the class.
    return _callHelper('callableClass(#, #)', [ctor, classExpr]);
  }

  /// Emits a constructor that ensures instances of this class are callable as
  /// functions in JavaScript.
  JS.Fun _emitCallableClassConstructor(ConstructorElement ctor) {
    return js.call(
        r'''function (...args) {
          function call(...args) {
            return call.call.apply(call, args);
          }
          call.__proto__ = this.__proto__;
          call.#.apply(call, args);
          return call;
        }''',
        [_constructorName(ctor)]);
  }

  void _emitClassTypeTests(ClassElement classElem, JS.Expression className,
      List<JS.Statement> body) {
    if (classElem == objectClass) {
      // We rely on ES6 static inheritance.  All types that are represented by
      // class constructor functions will see these definitions, with [this]
      // being bound to the class constructor.

      // The 'instanceof' checks don't work for primitive types (which have fast
      // definitions below) and don't work for native types. In those cases we
      // fall through to the general purpose checking code.
      body.add(js.statement(
          '#.is = function is_Object(o) {'
          '  if (o instanceof this) return true;'
          '  return #.is(o, this);'
          '}',
          [className, _runtimeModule]));
      body.add(js.statement(
          '#.as = function as_Object(o) {'
          '  if (o == null || o instanceof this) return o;'
          '  return #.as(o, this);'
          '}',
          [className, _runtimeModule]));
      body.add(js.statement(
          '#._check = function check_Object(o) {'
          '  if (o == null || o instanceof this) return o;'
          '  return #.check(o, this);'
          '}',
          [className, _runtimeModule]));
      return;
    }
    if (classElem == 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, #);'
          '}',
          [className, _runtimeModule, className]));
      body.add(js.statement(
          '#._check = function check_String(o) {'
          '  if (typeof o == "string" || o == null) return o;'
          '  return #.check(o, #);'
          '}',
          [className, _runtimeModule, className]));
      return;
    }
    if (classElem == 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, #);'
          '}',
          [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 #.check(o, #);'
          '}',
          [className, _runtimeModule, className]));
      return;
    }
    if (classElem == 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, #);'
          '}',
          [className, _runtimeModule, className]));
      body.add(js.statement(
          '#._check = function check_Null(o) {'
          '  if (o == null) return o;'
          '  return #.check(o, #);'
          '}',
          [className, _runtimeModule, className]));
      return;
    }
    if (classElem == numClass) {
      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, #);'
          '}',
          [className, _runtimeModule, className]));
      body.add(js.statement(
          '#._check = function check_num(o) {'
          '  if (typeof o == "number" || o == null) return o;'
          '  return #.check(o, #);'
          '}',
          [className, _runtimeModule, className]));
      return;
    }
    if (classElem == 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, #);'
          '}',
          [className, _runtimeModule, className]));
      body.add(js.statement(
          '#._check = function check_bool(o) {'
          '  if (o === true || o === false || o == null) return o;'
          '  return #.check(o, #);'
          '}',
          [className, _runtimeModule, className]));
      return;
    }
    // TODO(sra): Add special cases for hot tests like `x is html.Element`.

    // `instanceof` check is futile for classes that are Interceptor classes.
    ClassElement parent = classElem;
    while (parent != objectClass) {
      if (parent == interceptorClass) {
        if (classElem == interceptorClass) {
          // Place non-instanceof version of checks on Interceptor. All
          // interceptor classes will inherit the methods via ES6 class static
          // inheritance.
          body.add(_callHelperStatement('addTypeTests(#);', className));

          // TODO(sra): We could place on the extension type a pointer to the
          // peer constructor and use that for the `instanceof` check, e.g.
          //
          //    if (o instanceof this[_peerConstructor]) return o;
          //
        }
        return;
      }
      parent = parent.type.superclass.element;
    }

    // Choose between 'simple' checks, which are often accelerated by
    // `instanceof`, and other checks, which are slowed down by taking time to
    // do an `instanceof` check that is futile or likely futile.
    //
    // The `instanceof` check is futile for (1) a class that is only used as a
    // mixin, or (2) is only used as an interface in an `implements` clause, and
    // is likely futile (3) if the class has type parameters, since `Foo` aka
    // `Foo<dynamic>` is not a superclass of `Foo<int>`. The first two are
    // whole-program properites, but we can check for the last case.

    // Since ES6 classes have inheritance of static properties, we need only
    // install checks that differ from the parent.

    bool isSimple(ClassElement classElement) {
      if (classElement.typeParameters.isNotEmpty) return false;
      return true;
    }

    assert(classElem != objectClass);
    bool thisIsSimple = isSimple(classElem);
    bool superIsSimple = isSimple(classElem.type.superclass.element);

    if (thisIsSimple == superIsSimple) return;

    if (thisIsSimple) {
      body.add(_callHelperStatement('addSimpleTypeTests(#);', className));
    } else {
      body.add(_callHelperStatement('addTypeTests(#);', className));
    }
  }

  void _emitSuperHelperSymbols(
      List<JS.TemporaryId> superHelperSymbols, List<JS.Statement> body) {
    for (var id in superHelperSymbols) {
      body.add(js.statement('const # = Symbol(#)', [id, js.string(id.name)]));
    }
    superHelperSymbols.clear();
  }

  void _registerPropertyOverrides(
      ClassElement classElem,
      JS.Expression className,
      List<ClassElement> superclasses,
      List<FieldDeclaration> fields,
      Map<FieldElement, JS.TemporaryId> virtualFields,
      List<JS.Statement> virtualFieldSymbols,
      Set<FieldElement> staticFieldOverrides,
      Iterable<ExecutableElement> extensionMembers) {
    var extensionNames =
        new HashSet<String>.from(extensionMembers.map((e) => e.name));
    for (var field in fields) {
      for (VariableDeclaration fieldDecl in field.fields.variables) {
        var field = fieldDecl.element as FieldElement;
        var overrideInfo = checkForPropertyOverride(field, superclasses);
        if (overrideInfo.foundGetter ||
            overrideInfo.foundSetter ||
            extensionNames.contains(field.name)) {
          if (field.isStatic) {
            staticFieldOverrides.add(field);
          } else {
            var virtualField = new JS.TemporaryId(field.name);
            virtualFields[field] = virtualField;
            virtualFieldSymbols.add(js.statement(
                'const # = Symbol(#.name + "." + #.toString());',
                [virtualField, className, _declareMemberName(field.getter)]));
          }
        }
      }
    }
  }

  void _defineClass(ClassElement classElem, JS.Expression className,
      JS.ClassExpression classExpr, bool isCallable, List<JS.Statement> body) {
    JS.Expression callableClass;
    if (isCallable && classElem.unnamedConstructor != null) {
      callableClass =
          _emitCallableClass(classExpr, classElem.unnamedConstructor);
    }

    if (classElem.typeParameters.isNotEmpty) {
      if (callableClass != null) {
        body.add(js.statement('const # = #;', [classExpr.name, callableClass]));
      } else {
        body.add(new JS.ClassDeclaration(classExpr));
      }
    } else {
      body.add(js.statement('# = #;', [className, callableClass ?? classExpr]));
    }
  }

  List<JS.Identifier> _emitTypeFormals(List<TypeParameterElement> typeFormals) {
    return typeFormals
        .map((t) => new JS.Identifier(t.name))
        .toList(growable: false);
  }

  /// Emits a field declaration for TypeScript & Closure's ES6_TYPED
  /// (e.g. `class Foo { i: string; }`)
  JS.VariableDeclarationList _emitTypeScriptField(FieldDeclaration field) {
    return new JS.VariableDeclarationList(
        field.isStatic ? 'static' : null,
        field.fields.variables
            .map((decl) => new JS.VariableInitialization(
                new JS.Identifier(
                    // TODO(ochafik): use a refactored _emitMemberName instead.
                    decl.name.name,
                    type: emitTypeRef(decl.element.type)),
                null))
            .toList(growable: false));
  }

  @override
  JS.Statement visitEnumDeclaration(EnumDeclaration node) {
    var element = node.element;
    var type = element.type;

    // Generate a class per section 13 of the spec.
    // TODO(vsm): Generate any accompanying metadata

    // Create constructor and initialize index
    var constructor = new JS.Method(_propertyName('new'),
        js.call('function(index) { this.index = index; }') as JS.Fun);
    var fields = new List<FieldElement>.from(
        element.fields.where((f) => f.type == type));

    // Create toString() method
    var nameProperties = new List<JS.Property>(fields.length);
    for (var i = 0; i < fields.length; ++i) {
      nameProperties[i] = new JS.Property(
          js.number(i), js.string('${type.name}.${fields[i].name}'));
    }
    var nameMap = new JS.ObjectInitializer(nameProperties, multiline: true);
    var toStringF = new JS.Method(js.string('toString'),
        js.call('function() { return #[this.index]; }', nameMap) as JS.Fun);

    // Create enum class
    var classExpr = new JS.ClassExpression(new JS.Identifier(type.name),
        _emitClassHeritage(element), [constructor, toStringF]);
    var id = _emitTopLevelName(element);
    var result = [
      js.statement('# = #', [id, classExpr])
    ];

    // defineEnumValues internally depends on dart.constList which uses
    // _interceptors.JSArray.
    _declareBeforeUse(_jsArray);

    // Create static fields for each enum value, and the "values" getter
    result.add(_callHelperStatement('defineEnumValues(#, #);', [
      id,
      new JS.ArrayInitializer(fields.map((f) => _propertyName(f.name)).toList(),
          multiline: true)
    ]));

    return _statement(result);
  }

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

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

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

    if (element.library.isDartAsync &&
        (element.name == "Future" || element.name == "_Future")) {
      genericCall = _callHelper('flattenFutures(#)', [genericCall]);
    }
    var genericDef = js.statement(
        '# = #;', [_emitTopLevelName(element, suffix: r'$'), genericCall]);
    var dynType = fillDynamicTypeArgs(element.type);
    var genericInst = _emitType(dynType, lowerGeneric: true);
    return js.statement(
        '{ #; # = #; }', [genericDef, _emitTopLevelName(element), genericInst]);
  }

  bool _deferIfNeeded(DartType type, ClassElement current) {
    if (type is ParameterizedType) {
      var typeArguments = type.typeArguments;
      for (var typeArg in typeArguments) {
        var typeElement = typeArg.element;
        // FIXME(vsm): This does not track mutual recursive dependences.
        if (current == typeElement || _deferIfNeeded(typeArg, current)) {
          return true;
        }
      }
    }
    return false;
  }

  JS.ClassExpression _emitClassExpression(
      ClassElement element, List<JS.Method> methods,
      {List<FieldDeclaration> fields}) {
    String name = element.name;
    var heritage = _emitClassHeritage(element);
    var typeParams = _emitTypeFormals(element.typeParameters);
    var jsFields = fields?.map(_emitTypeScriptField)?.toList();

    return new JS.ClassExpression(new JS.Identifier(name), heritage, methods,
        typeParams: typeParams, fields: jsFields);
  }

  JS.Expression _emitClassHeritage(ClassElement element) {
    var type = element.type;
    if (type.isObject) return null;

    _loader.startTopLevel(element);

    // Find the super type
    JS.Expression heritage;
    var supertype = type.superclass;
    if (_deferIfNeeded(supertype, element)) {
      // Fall back to raw type.
      supertype = fillDynamicTypeArgs(supertype.element.type);
      _hasDeferredSupertype.add(element);
    }
    // We could choose to name the superclasses, but it's
    // not clear that there's much benefit
    heritage = _emitType(supertype, nameType: false);

    if (type.mixins.isNotEmpty) {
      var mixins =
          type.mixins.map((t) => _emitType(t, nameType: false)).toList();
      mixins.insert(0, heritage);
      heritage = _callHelper('mixin(#)', [mixins]);
    }

    _loader.finishTopLevel(element);

    return heritage;
  }

  /// 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(FieldDeclaration node) {
    // 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 (!node.isStatic) {
      for (var decl in node.fields.variables) {
        var field = decl.element as FieldElement;
        var name = getAnnotationName(field, isJsName) ?? field.name;
        // Generate getter
        var fn = new JS.Fun([], js.statement('{ return this.#; }', [name]));
        var method =
            new JS.Method(_declareMemberName(field.getter), fn, isGetter: true);
        jsMethods.add(method);

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

  List<JS.Method> _emitClassMethods(
      ClassDeclaration node,
      List<ConstructorDeclaration> ctors,
      List<FieldDeclaration> fields,
      List<ClassElement> superclasses,
      Map<FieldElement, JS.TemporaryId> virtualFields) {
    var element = node.element;
    var type = element.type;
    var isObject = type.isObject;

    // Iff no constructor is specified for a class C, it implicitly has a
    // default constructor `C() : super() {}`, unless C is class Object.
    var jsMethods = <JS.Method>[];
    if (isObject) {
      // Implements Dart constructor behavior.
      //
      // Because of ES6 constructor restrictions (`this` is not available until
      // `super` is called), we cannot emit an actual ES6 `constructor` on our
      // classes and preserve the Dart initialization order.
      //
      // Instead we use the same trick as named constructors, and do them as
      // instance methods that perform initialization.
      //
      // Therefore, dart:core Object gets the one real `constructor` and
      // immediately bounces to the `new() { ... }` initializer, letting us
      // bypass the ES6 restrictions.
      //
      // TODO(jmesserly): we'll need to rethink this.
      // See <https://github.com/dart-lang/dev_compiler/issues/51>.
      // This level of indirection will hurt performance.
      jsMethods.add(new JS.Method(
          _propertyName('constructor'),
          js.call('function(...args) { return this.new.apply(this, args); }')
          as JS.Fun));
    } else if (ctors.isEmpty) {
      jsMethods.add(_emitImplicitConstructor(node, fields, virtualFields));
    }

    bool hasJsPeer = findAnnotation(element, isJsPeerInterface) != null;

    bool hasIterator = false;
    for (var m in node.members) {
      if (m is ConstructorDeclaration) {
        jsMethods
            .add(_emitConstructor(m, type, fields, virtualFields, isObject));
      } else if (m is MethodDeclaration) {
        jsMethods.add(_emitMethodDeclaration(type, m));

        if (m.element is PropertyAccessorElement) {
          jsMethods.add(_emitSuperAccessorWrapper(m, type, superclasses));
        }

        if (!hasJsPeer && m.isGetter && m.name.name == 'iterator') {
          hasIterator = true;
          jsMethods.add(_emitIterable(type));
        }
      } else if (m is FieldDeclaration) {
        if (_extensionTypes.isNativeClass(element)) {
          jsMethods.addAll(_emitNativeFieldAccessors(m));
          continue;
        }
        if (m.isStatic) continue;
        for (VariableDeclaration field in m.fields.variables) {
          if (virtualFields.containsKey(field.element)) {
            jsMethods.addAll(_emitVirtualFieldAccessor(field, virtualFields));
          }
        }
      }
    }

    jsMethods.addAll(_implementMockInterfaces(type));

    // 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 && _implementsIterable(type)) {
      jsMethods.add(_emitIterable(type));
    }

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

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

  Iterable<ExecutableElement> _collectMockMethods(InterfaceType type) {
    var element = type.element;
    if (!_hasNoSuchMethod(element)) {
      return [];
    }

    // Collect all unimplemented members.
    //
    // Initially, we track abstract and concrete members separately, then
    // remove concrete from the abstract set. This is done because abstract
    // members are allowed to "override" concrete ones in Dart.
    // (In that case, it will still be treated as a concrete member and can be
    // called at run time.)
    var abstractMembers = new Map<String, ExecutableElement>();
    var concreteMembers = new HashSet<String>();

    void visit(InterfaceType type, bool isAbstract) {
      if (type == null) return;
      visit(type.superclass, isAbstract);
      for (var m in type.mixins) visit(m, isAbstract);
      for (var i in type.interfaces) visit(i, true);

      var members = <ExecutableElement>[]
        ..addAll(type.methods)
        ..addAll(type.accessors);
      for (var m in members) {
        if (isAbstract || m.isAbstract) {
          // Inconsistent signatures are disallowed, even with nSM, so we don't
          // need to worry too much about which abstract member we save.
          abstractMembers[m.name] = m;
        } else {
          concreteMembers.add(m.name);
        }
      }
    }

    visit(type, false);

    concreteMembers.forEach(abstractMembers.remove);
    return abstractMembers.values;
  }

  Iterable<JS.Method> _implementMockInterfaces(InterfaceType type) {
    // TODO(jmesserly): every type with nSM will generate new stubs for all
    // abstract members. For example:
    //
    //     class C { m(); noSuchMethod(...) { ... } }
    //     class D extends C { m(); noSuchMethod(...) { ... } }
    //
    // We'll generate D.m even though it is not necessary.
    //
    // Doing better is a bit tricky, as our current codegen strategy for the
    // mock methods encodes information about the number of arguments (and type
    // arguments) that D expects.
    return _collectMockMethods(type).map(_implementMockMethod);
  }

  /// 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('eatFood', args)));
  ///     }
  JS.Method _implementMockMethod(ExecutableElement method) {
    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 fnArgs = <JS.Parameter>[];
    JS.Expression positionalArgs;

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

    if (method is MethodElement) {
      addProperty('isMethod', js.boolean(true));

      fnArgs.add(new JS.RestParameter(args));
      positionalArgs = args;
    } else {
      var property = method as PropertyAccessorElement;
      if (property.isGetter) {
        addProperty('isGetter', js.boolean(true));

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

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

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

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

    var fn = new JS.Fun(fnArgs, js.statement('{ return #; }', [fnBody]),
        typeParams: _emitTypeFormals(method.type.typeFormals));

    // TODO(jmesserly): generic type arguments will get dropped.
    // We have a similar issue with `dgsend` helpers.
    return new JS.Method(
        _declareMemberName(method,
            useExtension:
                _extensionTypes.isNativeClass(method.enclosingElement)),
        _makeGenericFunction(fn),
        isGetter: method is PropertyAccessorElement && method.isGetter,
        isSetter: method is PropertyAccessorElement && method.isSetter,
        isStatic: false);
  }

  /// Return `true` if the given [classElement] has a noSuchMethod() method
  /// distinct from the one declared in class Object, as per the Dart Language
  /// Specification (section 10.4).
  // TODO(jmesserly): this was taken from error_verifier.dart
  bool _hasNoSuchMethod(ClassElement classElement) {
    // TODO(jmesserly): this is slow in Analyzer. It's a linear scan through all
    // methods, up through the class hierarchy.
    MethodElement method = classElement.lookUpMethod(
        FunctionElement.NO_SUCH_METHOD_METHOD_NAME, classElement.library);
    var definingClass = method?.enclosingElement;
    return definingClass != null && !definingClass.type.isObject;
  }

  /// 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(VariableDeclaration field,
      Map<FieldElement, JS.TemporaryId> virtualFields) {
    var virtualField = virtualFields[field.element];
    var result = <JS.Method>[];
    var name = _declareMemberName((field.element as FieldElement).getter);
    var getter = js.call('function() { return this[#]; }', [virtualField]);
    result.add(new JS.Method(name, getter, isGetter: true));

    if (field.isFinal) {
      var setter = js.call('function(value) { super[#] = value; }', [name]);
      result.add(new JS.Method(name, setter, isSetter: true));
    } else {
      var setter =
          js.call('function(value) { this[#] = value; }', [virtualField]);
      result.add(new JS.Method(name, setter, isSetter: true));
    }

    return result;
  }

  /// 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(MethodDeclaration method,
      InterfaceType type, List<ClassElement> superclasses) {
    var methodElement = method.element as PropertyAccessorElement;
    var field = methodElement.variable;
    if (!field.isSynthetic) return null;
    var propertyOverrideResult =
        checkForPropertyOverride(methodElement.variable, superclasses);

    // Generate a corresponding virtual getter / setter.
    var name = _declareMemberName(methodElement);
    if (method.isGetter) {
      // Generate a setter
      if (field.setter != null || !propertyOverrideResult.foundSetter)
        return null;
      var fn = js.call('function(value) { super[#] = value; }', [name]);
      return new JS.Method(name, fn, isSetter: true);
    } else {
      // Generate a getter
      if (field.getter != null || !propertyOverrideResult.foundGetter)
        return null;
      var fn = js.call('function() { return super[#]; }', [name]);
      return new JS.Method(name, fn, isGetter: true);
    }
  }

  bool _implementsIterable(InterfaceType t) =>
      t.interfaces.any((i) => i.element.type == types.iterableType);

  /// 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(InterfaceType t) {
    // 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 = t.lookUpGetterInSuperclass('iterator', t.element.library);
    if (parent != null) return null;
    var parentType = findSupertype(t, _implementsIterable);
    if (parentType != null) 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: t)]) as JS.Fun);
  }

  JS.Expression _instantiateAnnotation(Annotation node) {
    var element = node.element;
    if (element is ConstructorElement) {
      return _emitInstanceCreationExpression(element, element.returnType,
          node.constructorName, node.arguments, true);
    } else {
      return _visit(node.name);
    }
  }

  /// 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(ClassElement classElem) {
    var jsPeerNames = getAnnotationName(
        classElem,
        (a) =>
            isJsPeerInterface(a) ||
            isNativeAnnotation(a) && _extensionTypes.isNativeClass(classElem));
    if (jsPeerNames != null) {
      // Omit the special name "!nonleaf" and any future hacks starting with "!"
      return jsPeerNames
          .split(',')
          .where((peer) => !peer.startsWith("!"))
          .toList();
    } else {
      return [];
    }
  }

  void _registerExtensionType(
      ClassElement classElem, String jsPeerName, List<JS.Statement> body) {
    if (jsPeerName != null) {
      body.add(_callHelperStatement('registerExtension(#.global.#, #);', [
        _runtimeModule,
        _propertyName(jsPeerName),
        _emitTopLevelName(classElem)
      ]));
    }
  }

  JS.Statement _setBaseClass(ClassElement classElem, JS.Expression className,
      List<String> jsPeerNames, List<JS.Statement> body) {
    var typeFormals = classElem.typeParameters;
    if (jsPeerNames.isNotEmpty && typeFormals.isNotEmpty) {
      for (var peer in jsPeerNames) {
        // TODO(jmesserly): we should just extend Array in the first place
        var newBaseClass = _callHelper('global.#', [peer]);
        body.add(_callHelperStatement(
            'setExtensionBaseClass(#, #);', [className, newBaseClass]));
      }
    } else if (_hasDeferredSupertype.contains(classElem)) {
      var newBaseClass = _emitType(classElem.type.superclass,
          nameType: false, subClass: classElem, className: className);
      var deferredBaseClass = _callHelperStatement(
          'setBaseClass(#, #);', [className, newBaseClass]);
      if (typeFormals.isNotEmpty) return deferredBaseClass;
      body.add(deferredBaseClass);
    }
    return null;
  }

  void _defineNamedConstructors(List<ConstructorDeclaration> ctors,
      List<JS.Statement> body, JS.Expression className, bool isCallable) {
    var code = isCallable
        ? 'defineNamedConstructorCallable(#, #, #);'
        : 'defineNamedConstructor(#, #)';

    for (ConstructorDeclaration member in ctors) {
      if (member.name != null && member.factoryKeyword == null) {
        var args = [className, _constructorName(member.element)];
        if (isCallable) {
          args.add(_emitCallableClassConstructor(member.element));
        }

        body.add(_callHelperStatement(code, args));
      }
    }
  }

  /// Emits static fields for a class, and initialize them eagerly if possible,
  /// otherwise define them as lazy properties.
  void _emitStaticFields(
      List<FieldDeclaration> staticFields,
      Set<FieldElement> staticFieldOverrides,
      ClassElement classElem,
      List<JS.Statement> body) {
    var lazyStatics = <VariableDeclaration>[];
    for (FieldDeclaration member in staticFields) {
      for (VariableDeclaration field in member.fields.variables) {
        JS.Statement eagerField =
            _emitConstantStaticField(classElem, field, staticFieldOverrides);
        if (eagerField != null) {
          body.add(eagerField);
        } else {
          lazyStatics.add(field);
        }
      }
    }
    if (lazyStatics.isNotEmpty) {
      body.add(_emitLazyFields(classElem, lazyStatics));
    }
  }

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

  /// If a concrete class implements one of our extensions, we might need to
  /// add forwarders.
  void _defineExtensionMembers(List<ExecutableElement> extensions,
      JS.Expression className, List<JS.Statement> body) {
    // If a concrete class implements one of our extensions, we might need to
    // add forwarders.
    if (extensions.isNotEmpty) {
      var methodNames = <JS.Expression>[];
      for (var e in extensions) {
        methodNames.add(_declareMemberName(e, useExtension: false));
      }
      body.add(_callHelperStatement('defineExtensionMembers(#, #);', [
        className,
        new JS.ArrayInitializer(methodNames, multiline: methodNames.length > 4)
      ]));
    }
  }

  /// Emit the signature on the class recording the runtime type information
  void _emitClassSignature(
      List<MethodDeclaration> methods,
      List<FieldDeclaration> fields,
      ClassElement classElem,
      List<ConstructorDeclaration> ctors,
      List<ExecutableElement> extensions,
      JS.Expression className,
      List<JS.Statement> body) {
    if (classElem.interfaces.isNotEmpty) {
      body.add(js.statement('#[#.implements] = () => #;', [
        className,
        _runtimeModule,
        new JS.ArrayInitializer(
            new List<JS.Expression>.from(classElem.interfaces.map(_emitType)))
      ]));
    }

    var tStaticMethods = <JS.Property>[];
    var tInstanceMethods = <JS.Property>[];
    var tStaticGetters = <JS.Property>[];
    var tInstanceGetters = <JS.Property>[];
    var tStaticSetters = <JS.Property>[];
    var tInstanceSetters = <JS.Property>[];
    var sNames = <JS.Expression>[];
    for (MethodDeclaration node in methods) {
      var name = node.name.name;
      var element = node.element;
      // TODO(vsm): Clean up all the nasty duplication.
      if (node.isAbstract) {
        continue;
      }

      Function lookup;
      List<JS.Property> tMember;
      JS.Expression type;
      if (node.isGetter) {
        lookup = classElem.lookUpInheritedConcreteGetter;
        tMember = node.isStatic ? tStaticGetters : tInstanceGetters;
      } else if (node.isSetter) {
        lookup = classElem.lookUpInheritedConcreteSetter;
        tMember = node.isStatic ? tStaticSetters : tInstanceSetters;
      } else {
        // Method
        lookup = classElem.lookUpInheritedConcreteMethod;
        tMember = node.isStatic ? tStaticMethods : tInstanceMethods;
      }

      type = _emitAnnotatedFunctionType(element.type, node.metadata,
          parameters: node.parameters?.parameters,
          nameType: options.hoistSignatureTypes,
          hoistType: options.hoistSignatureTypes,
          definite: true);

      var inheritedElement = lookup(name, currentLibrary);
      if (inheritedElement != null && inheritedElement.type == element.type) {
        continue;
      }
      var memberName = _declareMemberName(element);
      var property = new JS.Property(memberName, type);
      tMember.add(property);
      // TODO(vsm): Why do we need this?
      if (node.isStatic && !node.isGetter && !node.isSetter) {
        sNames.add(memberName);
      }
    }

    var tInstanceFields = <JS.Property>[];
    var tStaticFields = <JS.Property>[];
    for (FieldDeclaration node in fields) {
      for (VariableDeclaration field in node.fields.variables) {
        var element = field.element as FieldElement;
        var memberName = _declareMemberName(element.getter);
        var type = _emitAnnotatedType(element.type, node.metadata);
        var property = new JS.Property(memberName, type);
        (node.isStatic ? tStaticFields : tInstanceFields).add(property);
      }
    }

    var tCtors = <JS.Property>[];
    for (ConstructorDeclaration node in ctors) {
      var memberName = _constructorName(node.element);
      var element = node.element;
      var type = _emitAnnotatedFunctionType(element.type, node.metadata,
          parameters: node.parameters.parameters,
          nameType: options.hoistSignatureTypes,
          hoistType: options.hoistSignatureTypes,
          definite: true);
      var property = new JS.Property(memberName, type);
      tCtors.add(property);
    }

    JS.Property build(String name, List<JS.Property> elements) {
      var o =
          new JS.ObjectInitializer(elements, multiline: elements.length > 1);
      // TODO(vsm): Remove
      var e = js.call('() => #', o);
      return new JS.Property(_propertyName(name), e);
    }

    var sigFields = <JS.Property>[];
    if (!tCtors.isEmpty) {
      sigFields.add(build('constructors', tCtors));
    }
    if (!tInstanceFields.isEmpty) {
      sigFields.add(build('fields', tInstanceFields));
    }
    if (!tInstanceGetters.isEmpty) {
      sigFields.add(build('getters', tInstanceGetters));
    }
    if (!tInstanceSetters.isEmpty) {
      sigFields.add(build('setters', tInstanceSetters));
    }
    if (!tInstanceMethods.isEmpty) {
      sigFields.add(build('methods', tInstanceMethods));
    }
    if (!tStaticFields.isEmpty) {
      sigFields.add(build('sfields', tStaticFields));
    }
    if (!tStaticGetters.isEmpty) {
      sigFields.add(build('sgetters', tStaticGetters));
    }
    if (!tStaticSetters.isEmpty) {
      sigFields.add(build('ssetters', tStaticSetters));
    }
    if (!tStaticMethods.isEmpty) {
      assert(!sNames.isEmpty);
      // TODO(vsm): Why do we need this names field?
      var aNames = new JS.Property(
          _propertyName('names'), new JS.ArrayInitializer(sNames));
      sigFields.add(build('statics', tStaticMethods));
      sigFields.add(aNames);
    }
    if (!sigFields.isEmpty || extensions.isNotEmpty) {
      var sig = new JS.ObjectInitializer(sigFields);
      body.add(_callHelperStatement('setSignature(#, #);', [className, sig]));
    }
    // Add static property dart._runtimeType to Object.
    // All other Dart classes will (statically) inherit this property.
    if (classElem == objectClass) {
      body.add(_callHelperStatement('tagComputed(#, () => #.#);',
          [className, emitLibraryName(dartCoreLibrary), 'Type']));
    }
  }

  /// Ensure `dartx.` symbols we will use are present.
  void _initExtensionSymbols(
      ClassElement classElem,
      List<MethodDeclaration> methods,
      List<FieldDeclaration> fields,
      List<JS.Statement> body) {
    if (_extensionTypes.hasNativeSubtype(classElem.type)) {
      var dartxNames = <JS.Expression>[];
      for (var m in methods) {
        if (!m.isAbstract && !m.isStatic && m.element.isPublic) {
          dartxNames.add(_declareMemberName(m.element, useExtension: false));
        }
      }
      for (var fieldDecl in fields) {
        if (!fieldDecl.isStatic) {
          for (var field in fieldDecl.fields.variables) {
            var e = field.element as FieldElement;
            if (e.isPublic) {
              dartxNames.add(_declareMemberName(e.getter, useExtension: false));
            }
          }
        }
      }
      if (dartxNames.isNotEmpty) {
        body.add(_callHelperStatement('defineExtensionNames(#)',
            [new JS.ArrayInitializer(dartxNames, multiline: true)]));
      }
    }
  }

  List<ExecutableElement> _extensionsToImplement(ClassElement element) {
    var members = <ExecutableElement>[];
    if (_extensionTypes.isNativeClass(element)) return members;

    // Collect all extension types we implement.
    var type = element.type;
    var types = _extensionTypes.collectNativeInterfaces(element);
    if (types.isEmpty) return members;

    // Collect all possible extension method names.
    var extensionMembers = new HashSet<String>();
    for (var t in types) {
      for (var m in [t.methods, t.accessors].expand((e) => e)) {
        if (!m.isStatic && m.isPublic) extensionMembers.add(m.name);
      }
    }

    // Collect all of extension methods this type implements.
    for (var m in [type.methods, type.accessors].expand((e) => e)) {
      if (!m.isStatic && !m.isAbstract && extensionMembers.contains(m.name)) {
        members.add(m);
      }
    }
    return members;
  }

  /// Generates the implicit default constructor for class C of the form
  /// `C() : super() {}`.
  JS.Method _emitImplicitConstructor(
      ClassDeclaration node,
      List<FieldDeclaration> fields,
      Map<FieldElement, JS.TemporaryId> virtualFields) {
    // If we don't have a method body, skip this.
    var superCall = _superConstructorCall(node.element);
    if (fields.isEmpty && superCall == null) return null;

    var initFields = _initializeFields(node, fields, virtualFields);
    List<JS.Statement> body = [initFields];
    if (superCall != null) {
      body.add(superCall);
    }
    var name = _constructorName(node.element.unnamedConstructor);
    return annotate(
        new JS.Method(name, js.call('function() { #; }', [body]) as JS.Fun),
        node,
        node.element);
  }

  JS.Method _emitConstructor(
      ConstructorDeclaration node,
      InterfaceType type,
      List<FieldDeclaration> fields,
      Map<FieldElement, JS.TemporaryId> virtualFields,
      bool isObject) {
    if (_externalOrNative(node)) return null;

    var name = _constructorName(node.element);
    var returnType = emitTypeRef(node.element.enclosingElement.type);

    // Wacky factory redirecting constructors: factory Foo.q(x, y) = Bar.baz;
    var redirect = node.redirectedConstructor;
    if (redirect != null) {
      var newKeyword = redirect.staticElement.isFactory ? '' : 'new';
      // Pass along all arguments verbatim, and let the callee handle them.
      // TODO(jmesserly): we'll need something different once we have
      // rest/spread support, but this should work for now.
      var params =
          visitFormalParameterList(node.parameters, destructure: false);

      var fun = new JS.Fun(
          params,
          js.statement('{ return $newKeyword #(#); }',
              [_visit(redirect) as JS.Node, params]),
          returnType: returnType);
      return annotate(
          new JS.Method(name, fun, isStatic: true), node, node.element);
    }

    // For const constructors we need to ensure default values are
    // available for use by top-level constant initializers.
    ClassDeclaration cls = node.parent;
    if (node.constKeyword != null) _loader.startTopLevel(cls.element);
    var params = visitFormalParameterList(node.parameters);
    if (node.constKeyword != null) _loader.finishTopLevel(cls.element);

    // Factory constructors are essentially static methods.
    if (node.factoryKeyword != null) {
      var body = <JS.Statement>[];
      var init = _emitArgumentInitializers(node, constructor: true);
      if (init != null) body.add(init);
      body.add(_visit(node.body));
      var fun = new JS.Fun(params, new JS.Block(body), returnType: returnType);
      return annotate(
          new JS.Method(name, fun, isStatic: true), node, node.element);
    }

    // Code generation for Object's constructor.
    var savedFunction = _currentFunction;
    _currentFunction = node.body;
    var body = _emitConstructorBody(node, fields, virtualFields);
    _currentFunction = savedFunction;

    // We generate constructors as initializer methods in the class;
    // this allows use of `super` for instance methods/properties.
    // It also avoids V8 restrictions on `super` in default constructors.
    return annotate(
        new JS.Method(name, new JS.Fun(params, body, returnType: returnType)),
        node,
        node.element);
  }

  JS.Expression _constructorName(ConstructorElement ctor) {
    var name = ctor.name;
    if (name == '') {
      // Default constructors (factory or not) use `new` as their name.
      return _propertyName('new');
    }
    return _emitMemberName(name, isStatic: true);
  }

  JS.Block _emitConstructorBody(
      ConstructorDeclaration node,
      List<FieldDeclaration> fields,
      Map<FieldElement, JS.TemporaryId> virtualFields) {
    var body = <JS.Statement>[];
    ClassDeclaration cls = node.parent;

    // 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.
    if (node.constKeyword != null) _loader.startTopLevel(cls.element);
    var init = _emitArgumentInitializers(node, constructor: true);
    if (node.constKeyword != null) _loader.finishTopLevel(cls.element);
    if (init != null) body.add(init);

    // 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 RedirectingConstructorInvocation,
        orElse: () => null);

    if (redirectCall != null) {
      body.add(_visit(redirectCall));
      return new JS.Block(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(cls, fields, virtualFields, node));

    var superCall = node.initializers.firstWhere(
        (i) => i is SuperConstructorInvocation,
        orElse: () => null) as SuperConstructorInvocation;

    // 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 jsSuper = _superConstructorCall(cls.element, superCall);
    if (jsSuper != null) body.add(jsSuper);

    body.add(_visit(node.body));
    return new JS.Block(body)..sourceInformation = node;
  }

  @override
  JS.Statement visitRedirectingConstructorInvocation(
      RedirectingConstructorInvocation node) {
    var ctor = node.staticElement;
    var cls = ctor.enclosingElement;
    // 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('#.prototype.#.call(this, #);', [
      new JS.Identifier(cls.name),
      _constructorName(ctor),
      _visit(node.argumentList)
    ]);
  }

  JS.Statement _superConstructorCall(ClassElement element,
      [SuperConstructorInvocation node]) {
    if (element.supertype == null) {
      assert(element.type.isObject || options.unsafeForceCompile);
      return null;
    }

    ConstructorElement superCtor;
    if (node != null) {
      superCtor = node.staticElement;
    } else {
      // Get the supertype's unnamed constructor.
      superCtor = element.supertype.element.unnamedConstructor;
    }

    if (superCtor == null) {
      // This will only happen if the code has errors:
      // we're trying to generate an implicit constructor for a type where
      // we don't have a default constructor in the supertype.
      assert(options.unsafeForceCompile);
      return null;
    }

    if (superCtor.name == '' && !_hasUnnamedSuperConstructor(element)) {
      return null;
    }

    var name = _constructorName(superCtor);
    var args = node != null ? _visit(node.argumentList) : [];
    return annotate(js.statement('super.#(#);', [name, args]), node);
  }

  bool _hasUnnamedSuperConstructor(ClassElement e) {
    var supertype = e.supertype;
    if (supertype == null) return false;
    if (_hasUnnamedConstructor(supertype.element)) return true;
    for (var mixin in e.mixins) {
      if (_hasUnnamedConstructor(mixin.element)) return true;
    }
    return false;
  }

  bool _hasUnnamedConstructor(ClassElement e) {
    if (e.type.isObject) return false;
    if (!e.unnamedConstructor.isSynthetic) return true;
    if (e.fields.any((f) => !f.isStatic && !f.isSynthetic)) return true;
    return _hasUnnamedSuperConstructor(e);
  }

  /// 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(
      ClassDeclaration cls,
      List<FieldDeclaration> fieldDecls,
      Map<FieldElement, JS.TemporaryId> virtualFields,
      [ConstructorDeclaration ctor]) {
    bool isConst = ctor != null && ctor.constKeyword != null;
    if (isConst) _loader.startTopLevel(cls.element);

    // Run field initializers if they can have side-effects.
    var fields = new Map<FieldElement, JS.Expression>();
    var unsetFields = new Map<FieldElement, VariableDeclaration>();
    for (var declaration in fieldDecls) {
      for (var fieldNode in declaration.fields.variables) {
        var element = fieldNode.element;
        if (_constants.isFieldInitConstant(fieldNode)) {
          unsetFields[element as FieldElement] = fieldNode;
        } else {
          fields[element as FieldElement] = _visitInitializer(fieldNode);
        }
      }
    }

    // Initialize fields from `this.fieldName` parameters.
    if (ctor != null) {
      for (var p in ctor.parameters.parameters) {
        var element = p.element;
        if (element is FieldFormalParameterElement) {
          fields[element.field] = _emitSimpleIdentifier(p.identifier);
        }
      }

      // Run constructor field initializers such as `: foo = bar.baz`
      for (var init in ctor.initializers) {
        if (init is ConstructorFieldInitializer) {
          fields[init.fieldName.staticElement as FieldElement] =
              _visit(init.expression);
        }
      }
    }

    for (var f in fields.keys) unsetFields.remove(f);

    // Initialize all remaining fields
    unsetFields.forEach((element, fieldNode) {
      JS.Expression value;
      if (fieldNode.initializer != null) {
        value = _visit(fieldNode.initializer);
      } else {
        value = new JS.LiteralNull();
      }
      fields[element] = value;
    });

    var body = <JS.Statement>[];
    fields.forEach((FieldElement e, JS.Expression initialValue) {
      JS.Expression access = virtualFields[e] ?? _declareMemberName(e.getter);
      body.add(js.statement('this.# = #;', [access, initialValue]));
    });

    if (isConst) _loader.finishTopLevel(cls.element);
    return _statement(body);
  }

  FormalParameterList _parametersOf(node) {
    // TODO(jmesserly): clean this up. If we can model ES6 spread/rest args, we
    // could handle argument initializers more consistently in a separate
    // lowering pass.
    if (node is ConstructorDeclaration) return node.parameters;
    if (node is MethodDeclaration) return node.parameters;
    if (node is FunctionDeclaration) node = node.functionExpression;
    return (node as FunctionExpression).parameters;
  }

  /// Emits argument initializers, which handles optional/named args, as well
  /// as generic type checks needed due to our covariance.
  JS.Statement _emitArgumentInitializers(node, {bool constructor: false}) {
    // Constructor argument initializers are emitted earlier in the code, rather
    // than always when we visit the function body, so we control it explicitly.
    if (node is ConstructorDeclaration != constructor) return null;

    var parameters = _parametersOf(node);
    if (parameters == null) return null;

    var body = <JS.Statement>[];
    for (var param in parameters.parameters) {
      var jsParam = _emitSimpleIdentifier(param.identifier);

      if (!options.destructureNamedParams) {
        if (param.kind == ParameterKind.NAMED) {
          // Parameters will be passed using their real names, not the (possibly
          // renamed) local variable.
          var paramName = js.string(param.identifier.name, "'");

          // TODO(ochafik): Fix `'prop' in obj` to please Closure's renaming.
          body.add(js.statement('let # = # && # in # ? #.# : #;', [
            jsParam,
            namedArgumentTemp,
            paramName,
            namedArgumentTemp,
            namedArgumentTemp,
            paramName,
            _defaultParamValue(param),
          ]));
        } else if (param.kind == ParameterKind.POSITIONAL) {
          body.add(js.statement('if (# === void 0) # = #;',
              [jsParam, jsParam, _defaultParamValue(param)]));
        }
      }

      // TODO(jmesserly): various problems here, see:
      // https://github.com/dart-lang/dev_compiler/issues/116
      var paramType = param.element.type;
      if (node is MethodDeclaration &&
          (param.element.isCovariant || _unsoundCovariant(paramType, true)) &&
          !_inWhitelistCode(node)) {
        var castType = _emitType(paramType,
            nameType: options.nameTypeTests || options.hoistTypeTests,
            hoistType: options.hoistTypeTests);
        body.add(js.statement('#._check(#);', [castType, jsParam]));
      }
    }
    return body.isEmpty ? null : _statement(body);
  }

  /// Given a type [t], return whether or not t is unsoundly covariant.
  /// If [contravariant] is true, then t appears in a contravariant
  /// position.
  bool _unsoundCovariant(DartType t, bool contravariant) {
    if (t is TypeParameterType) {
      return contravariant && t.element.enclosingElement is ClassElement;
    }
    if (t is FunctionType) {
      if (_unsoundCovariant(t.returnType, contravariant)) return true;
      return t.parameters.any((p) => _unsoundCovariant(p.type, !contravariant));
    }
    if (t is ParameterizedType) {
      return t.typeArguments.any((t) => _unsoundCovariant(t, contravariant));
    }
    return false;
  }

  JS.Expression _defaultParamValue(FormalParameter param) {
    if (param is DefaultFormalParameter && param.defaultValue != null) {
      return _visit(param.defaultValue);
    } else {
      return new JS.LiteralNull();
    }
  }

  JS.Fun _emitNativeFunctionBody(MethodDeclaration node) {
    String name =
        getAnnotationName(node.element, isJSAnnotation) ?? node.name.name;
    if (node.isGetter) {
      return new JS.Fun([], js.statement('{ return this.#; }', [name]));
    } else if (node.isSetter) {
      var params =
          visitFormalParameterList(node.parameters, destructure: false);
      return new JS.Fun(
          params, js.statement('{ this.# = #; }', [name, params.last]));
    } else {
      return js.call(
          'function (...args) { return this.#.apply(this, args); }', name);
    }
  }

  JS.Method _emitMethodDeclaration(InterfaceType type, MethodDeclaration node) {
    if (node.isAbstract) {
      return null;
    }

    JS.Fun fn;
    if (_externalOrNative(node)) {
      if (node.isStatic) {
        // TODO(vsm): Do we need to handle this case?
        return null;
      }
      fn = _emitNativeFunctionBody(node);
    } else {
      fn = _emitFunctionBody(node.element, node.parameters, node.body);

      if (node.operatorKeyword != null &&
          node.name.name == '[]=' &&
          fn.params.isNotEmpty) {
        // []= methods need to return the value. We could also address this at
        // call sites, but it's cleaner to instead transform the operator method.
        fn = _alwaysReturnLastParameter(fn);
      }

      fn = _makeGenericFunction(fn);
    }

    return annotate(
        new JS.Method(_declareMemberName(node.element), fn,
            isGetter: node.isGetter,
            isSetter: node.isSetter,
            isStatic: node.isStatic),
        node,
        node.element);
  }

  /// Transform the function so the last parameter is always returned.
  ///
  /// This is useful for indexed set methods, which otherwise would not have
  /// the right return value in JS.
  JS.Fun _alwaysReturnLastParameter(JS.Fun fn) {
    var body = fn.body;
    if (JS.Return.foundIn(fn)) {
      // If a return is inside body, transform `(params) { body }` to
      // `(params) { (() => { body })(); return value; }`.
      // TODO(jmesserly): we could instead generate the return differently,
      // and avoid the immediately invoked function.
      body = new JS.Call(new JS.ArrowFun([], fn.body), []).toStatement();
    }
    // Rewrite the function to include the return.
    return new JS.Fun(
        fn.params, new JS.Block([body, new JS.Return(fn.params.last)]),
        typeParams: fn.typeParams,
        returnType: fn.returnType)..sourceInformation = fn.sourceInformation;
  }

  @override
  JS.Statement visitFunctionDeclaration(FunctionDeclaration node) {
    assert(node.parent is CompilationUnit);

    if (_externalOrNative(node)) return null;

    // If we have a getter/setter pair, they need to be defined together.
    if (node.isGetter) {
      PropertyAccessorElement element = node.element;
      var props = <JS.Method>[_emitTopLevelProperty(node)];
      var setter = element.correspondingSetter;
      if (setter != null) {
        props.add(_loader.emitDeclaration(
            setter, (node) => _emitTopLevelProperty(node)));
      }
      return _callHelperStatement('copyProperties(#, { # });',
          [emitLibraryName(currentLibrary), props]);
    }
    if (node.isSetter) {
      PropertyAccessorElement element = node.element;
      var props = <JS.Method>[_emitTopLevelProperty(node)];
      var getter = element.correspondingGetter;
      if (getter != null) {
        props.add(_loader.emitDeclaration(
            getter, (node) => _emitTopLevelProperty(node)));
      }
      return _callHelperStatement('copyProperties(#, { # });',
          [emitLibraryName(currentLibrary), props]);
    }

    var body = <JS.Statement>[];
    var fn = _emitFunction(node.functionExpression);

    if (currentLibrary.source.isInSystemLibrary &&
        _isInlineJSFunction(node.functionExpression)) {
      fn = _simplifyPassThroughArrowFunCallBody(fn);
    }

    var element = node.element;
    var nameExpr = _emitTopLevelName(element);
    body.add(annotate(js.statement('# = #', [nameExpr, fn]), node, element));
    if (!_isDartRuntime(element.library)) {
      body.add(_emitFunctionTagged(nameExpr, element.type, topLevel: true)
          .toStatement());
    }

    return _statement(body);
  }

  bool _isInlineJSFunction(FunctionExpression functionExpression) {
    var body = functionExpression.body;
    if (body is ExpressionFunctionBody) {
      return _isJSInvocation(body.expression);
    } else if (body is BlockFunctionBody) {
      var statements = body.block.statements;
      if (statements.length == 1) {
        var stat = statements[0];
        if (stat is ReturnStatement) {
          return _isJSInvocation(stat.expression);
        }
      }
    }
    return false;
  }

  bool _isJSInvocation(Expression expr) =>
      expr is MethodInvocation && isInlineJS(expr.methodName.staticElement);

  // Simplify `(args) => (() => { ... })()` to `(args) => { ... }`.
  // Note: this allows silently passing args through to the body, which only
  // works if we don't do weird renamings of Dart params.
  JS.Fun _simplifyPassThroughArrowFunCallBody(JS.Fun fn) {
    if (fn.body is JS.Block && fn.body.statements.length == 1) {
      var stat = fn.body.statements.single;
      if (stat is JS.Return && stat.value is JS.Call) {
        JS.Call call = stat.value;
        if (call.target is JS.ArrowFun && call.arguments.isEmpty) {
          JS.ArrowFun innerFun = call.target;
          if (innerFun.params.isEmpty) {
            return new JS.Fun(fn.params, innerFun.body,
                typeParams: fn.typeParams, returnType: fn.returnType);
          }
        }
      }
    }
    return fn;
  }

  JS.Method _emitTopLevelProperty(FunctionDeclaration node) {
    var name = node.name.name;
    return annotate(
        new JS.Method(
            _propertyName(name), _emitFunction(node.functionExpression),
            isGetter: node.isGetter, isSetter: node.isSetter),
        node,
        node.element);
  }

  bool _executesAtTopLevel(AstNode node) {
    var ancestor = node.getAncestor((n) =>
        n is FunctionBody ||
        (n is FieldDeclaration && n.staticKeyword == null) ||
        (n is ConstructorDeclaration && n.constKeyword == null));
    return ancestor == null;
  }

  bool _typeIsLoaded(DartType type) {
    if (type is FunctionType && (type.name == '' || type.name == null)) {
      return (_typeIsLoaded(type.returnType) &&
          type.optionalParameterTypes.every(_typeIsLoaded) &&
          type.namedParameterTypes.values.every(_typeIsLoaded) &&
          type.normalParameterTypes.every(_typeIsLoaded));
    }
    if (type.isDynamic || type.isVoid || type.isBottom) return true;
    if (type is ParameterizedType && !type.typeArguments.every(_typeIsLoaded)) {
      return false;
    }
    return _loader.isLoaded(type.element);
  }

  JS.Expression _emitFunctionTagged(JS.Expression fn, DartType type,
      {topLevel: false}) {
    var lazy = topLevel && !_typeIsLoaded(type);
    var typeRep = _emitFunctionType(type, definite: true);
    if (lazy) {
      return _callHelper('lazyFn(#, () => #)', [fn, typeRep]);
    } else {
      return _callHelper('fn(#, #)', [fn, typeRep]);
    }
  }

  /// Emits an arrow FunctionExpression node.
  ///
  /// This should be used for all places in Dart's AST where FunctionExpression
  /// appears and the function is actually in an Expression context. These
  /// correspond to arrow functions in Dart.
  ///
  /// Contrast with [_emitFunction].
  @override
  JS.Expression visitFunctionExpression(FunctionExpression node) {
    assert(node.parent is! FunctionDeclaration &&
        node.parent is! MethodDeclaration);
    return _emitFunctionTagged(_emitArrowFunction(node), getStaticType(node),
        topLevel: _executesAtTopLevel(node));
  }

  JS.ArrowFun _emitArrowFunction(FunctionExpression node) {
    JS.Fun f = _emitFunctionBody(node.element, node.parameters, node.body);
    JS.Node body = f.body;

    // Simplify `=> { return e; }` to `=> e`
    if (body is JS.Block) {
      JS.Block block = body;
      if (block.statements.length == 1) {
        JS.Statement s = block.statements[0];
        if (s is JS.Return && s.value != null) body = s.value;
      }
    }

    // Convert `function(...) { ... }` to `(...) => ...`
    // This is for readability, but it also ensures correct `this` binding.
    var fn = new JS.ArrowFun(f.params, body,
        typeParams: f.typeParams, returnType: f.returnType);

    return annotate(_makeGenericArrowFun(fn), node);
  }

  JS.ArrowFun _makeGenericArrowFun(JS.ArrowFun fn) {
    if (fn.typeParams == null || fn.typeParams.isEmpty) return fn;
    return new JS.ArrowFun(fn.typeParams, fn);
  }

  JS.Fun _makeGenericFunction(JS.Fun fn) {
    if (fn.typeParams == null || fn.typeParams.isEmpty) return fn;

    // TODO(jmesserly): we could make these default to `dynamic`.
    return new JS.Fun(
        fn.typeParams,
        new JS.Block([
          // Convert the function to an => function, to ensure `this` binding.
          new JS.Return(new JS.ArrowFun(fn.params, fn.body,
              typeParams: fn.typeParams, returnType: fn.returnType))
        ]));
  }

  /// Emits a non-arrow FunctionExpression node.
  ///
  /// This should be used for all places in Dart's AST where FunctionExpression
  /// appears but the function is not actually in an Expression context, such
  /// as methods, properties, and top-level functions.
  ///
  /// Contrast with [visitFunctionExpression].
  JS.Fun _emitFunction(FunctionExpression node) {
    var fn = _emitFunctionBody(node.element, node.parameters, node.body);
    return annotate(_makeGenericFunction(fn), node);
  }

  JS.Fun _emitFunctionBody(ExecutableElement element,
      FormalParameterList parameters, FunctionBody body) {
    FunctionType type = element.type;

    // normal function (sync), vs (sync*, async, async*)
    var stdFn = !(element.isAsynchronous || element.isGenerator);
    var formals = visitFormalParameterList(parameters, destructure: stdFn);
    var code = (stdFn)
        ? _visit(body)
        : new JS.Block(
            [_emitGeneratorFunctionBody(element, parameters, body).toReturn()]);
    var typeFormals = _emitTypeFormals(type.typeFormals);
    var returnType = emitTypeRef(type.returnType);
    if (type.typeFormals.isNotEmpty) {
      code = new JS.Block(<JS.Statement>[
        new JS.Block(_typeTable.discharge(type.typeFormals)),
        code
      ]);
    }
    return new JS.Fun(formals, code,
        typeParams: typeFormals, returnType: returnType);
  }

  JS.Expression _emitGeneratorFunctionBody(ExecutableElement element,
      FormalParameterList parameters, FunctionBody body) {
    var kind = element.isSynchronous ? 'sync' : 'async';
    if (element.isGenerator) kind += 'Star';

    // Transforms `sync*` `async` and `async*` function bodies
    // using ES6 generators.
    //
    // `sync*` wraps a generator in a Dart Iterable<T>:
    //
    // function name(<args>) {
    //   return dart.syncStar(function*(<args>) {
    //     <body>
    //   }, T, <args>).bind(this);
    // }
    //
    // We need to include <args> in case any are mutated, so each `.iterator`
    // gets the same initial values.
    //
    // TODO(jmesserly): we could omit the args for the common case where args
    // are not mutated inside the generator.
    //
    // In the future, we might be able to simplify this, see:
    // https://github.com/dart-lang/dev_compiler/issues/247.
    //
    // `async` works the same, but uses the `dart.async` helper.
    //
    // In the body of a `sync*` and `async`, `yield`/`await` are both generated
    // simply as `yield`.
    //
    // `async*` uses the `dart.asyncStar` helper, and also has an extra `stream`
    // argument to the generator, which is used for passing values to the
    // _AsyncStarStreamController implementation type.
    // `yield` is specially generated inside `async*`, see visitYieldStatement.
    // `await` is generated as `yield`.
    // runtime/_generators.js has an example of what the code is generated as.
    var savedController = _asyncStarController;
    var jsParams = visitFormalParameterList(parameters);
    if (kind == 'asyncStar') {
      _asyncStarController = new JS.TemporaryId('stream');
      jsParams.insert(0, _asyncStarController);
    } else {
      _asyncStarController = null;
    }
    var savedSuperAllowed = _superAllowed;
    _superAllowed = false;
    // Visit the body with our async* controller set.
    var jsBody = _visit(body);
    _superAllowed = savedSuperAllowed;
    _asyncStarController = savedController;

    DartType returnType = _getExpectedReturnType(element);
    JS.Expression gen = new JS.Fun(jsParams, jsBody,
        isGenerator: true, returnType: emitTypeRef(returnType));
    if (JS.This.foundIn(gen)) {
      gen = js.call('#.bind(this)', gen);
    }

    var T = _emitType(returnType);
    return _callHelper('#(#)', [
      kind,
      [gen, T]..addAll(visitFormalParameterList(parameters, destructure: false))
    ]);
  }

  @override
  JS.Statement visitFunctionDeclarationStatement(
      FunctionDeclarationStatement node) {
    var func = node.functionDeclaration;
    if (func.isGetter || func.isSetter) {
      return js.comment('Unimplemented function get/set statement: $node');
    }

    var fn = _emitFunction(func.functionExpression);

    var name = new JS.Identifier(func.name.name);
    JS.Statement declareFn;
    if (JS.This.foundIn(fn)) {
      declareFn = js.statement('const # = #.bind(this);', [name, fn]);
    } else {
      declareFn = new JS.FunctionDeclaration(name, fn);
    }
    declareFn = annotate(declareFn, node, node.functionDeclaration.element);

    return new JS.Block([
      declareFn,
      _emitFunctionTagged(name, func.element.type).toStatement()
    ]);
  }

  /// Emits a simple identifier, including handling an inferred generic
  /// function instantiation.
  @override
  JS.Expression visitSimpleIdentifier(SimpleIdentifier node) {
    var typeArgs = _getTypeArgs(node.staticElement, node.staticType);
    var simpleId = _emitSimpleIdentifier(node);
    if (typeArgs == null) {
      return simpleId;
    }
    return _callHelper('gbind(#, #)', [simpleId, typeArgs]);
  }

  /// Emits a simple identifier, handling implicit `this` as well as
  /// going through the qualified library name if necessary, but *not* handling
  /// inferred generic function instantiation.
  JS.Expression _emitSimpleIdentifier(SimpleIdentifier node) {
    var accessor = node.staticElement;
    if (accessor == null) {
      return js.commentExpression(
          'Unimplemented unknown name', new JS.Identifier(node.name));
    }

    // Get the original declaring element. If we had a property accessor, this
    // indirects back to a (possibly synthetic) field.
    var element = accessor;
    if (accessor is PropertyAccessorElement) element = accessor.variable;

    _declareBeforeUse(element);

    // type literal
    if (element is TypeDefiningElement) {
      var typeName = _emitType(fillDynamicTypeArgs(element.type));

      // If the type is a type literal expression in Dart code, wrap the raw
      // runtime type in a "Type" instance.
      if (!_isInForeignJS && _isTypeLiteral(node)) {
        typeName = _callHelper('wrapType(#)', typeName);
      }

      return typeName;
    }

    // library member
    if (element.enclosingElement is CompilationUnitElement) {
      return _emitTopLevelName(element);
    }

    var name = element.name;

    // Unqualified class member. This could mean implicit-this, or implicit
    // call to a static from the same class.
    if (element is ClassMemberElement && element is! ConstructorElement) {
      bool isStatic = element.isStatic;
      var type = element.enclosingElement.type;
      var member = _emitMemberName(name,
          isStatic: isStatic, type: type, element: element);

      if (isStatic) {
        var dynType = _emitStaticAccess(type);
        return new JS.PropertyAccess(dynType, member);
      }

      // For instance members, we add implicit-this.
      // For method tear-offs, we ensure it's a bound method.
      var tearOff = element is MethodElement && !inInvocationContext(node);
      if (tearOff) return _callHelper('bind(this, #)', member);
      return js.call('this.#', member);
    }

    if (element is ParameterElement) {
      return _emitParameter(element);
    }

    // If this is one of our compiler's temporary variables, return its JS form.
    if (element is TemporaryVariableElement) {
      return element.jsVariable;
    }

    return new JS.Identifier(name);
  }

  /// Returns `true` if the type name referred to by [node] is used in a
  /// position where it should evaluate as a type literal -- an object of type
  /// Type.
  bool _isTypeLiteral(SimpleIdentifier node) {
    var parent = node.parent;

    // Static member call.
    if (parent is MethodInvocation || parent is PropertyAccess) return false;

    // An expression like "a.b".
    if (parent is PrefixedIdentifier) {
      // In "a.b", "b" may be a type literal, but "a", is not.
      if (node != parent.identifier) return false;

      // If the prefix expression is itself used as an invocation, like
      // "a.b.c", then "b" is not a type literal.
      var grand = parent.parent;
      if (grand is MethodInvocation || grand is PropertyAccess) return false;

      return true;
    }

    // In any other context, it's a type literal.
    return true;
  }

  JS.Identifier _emitParameter(ParameterElement element,
      {bool declaration: false}) {
    // initializing formal parameter, e.g. `Point(this._x)`
    // TODO(jmesserly): type ref is not attached in this case.
    if (element.isInitializingFormal && element.isPrivate) {
      /// Rename private names so they don't shadow the private field symbol.