Report error on JS-interop operator [] and []= methods.

pkg/compiler/lib/src/common/backend_api.dart

 // Copyright (c) 2012, 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.
 
 library dart2js.backend_api;
 
 import 'dart:async' show Future;
 
 import '../common.dart';
 import '../common/codegen.dart' show
@@ skipping 13 matching lines @@
     ConstantValue;
 import '../dart_types.dart' show
     DartType,
     InterfaceType;
 import '../elements/elements.dart' show
     ClassElement,
     ConstructorElement,
     Element,
     FunctionElement,
     LibraryElement,
-    MetadataAnnotation;
+    MetadataAnnotation,
+    MethodElement;
 import '../enqueue.dart' show
     Enqueuer,
     CodegenEnqueuer,
     ResolutionEnqueuer;
 import '../io/code_output.dart' show
     CodeBuffer;
 import '../io/source_information.dart' show
     SourceInformationStrategy;
 import '../js_backend/backend_helpers.dart' as js_backend show
     BackendHelpers;
@@ skipping 230 matching lines @@
   /// Returns `true` if [element] is implemented via typed JavaScript interop.
   // TODO(johnniwinther): Move this to [JavaScriptBackend].
   bool isJsInterop(Element element) => false;
 
   /// Returns `true` if the `native` pseudo keyword is supported for [library].
   bool canLibraryUseNative(LibraryElement library) {
     // TODO(johnniwinther): Move this to [JavaScriptBackend].
     return native.maybeEnableNative(compiler, library);
   }
 
-  /// Processes [element] for resolution and returns the [FunctionElement] that
+  /// Processes [element] for resolution and returns the [MethodElement] that
   /// defines the implementation of [element].
-  FunctionElement resolveExternalFunction(FunctionElement element) => element;
+  MethodElement resolveExternalFunction(MethodElement element) => element;
 
   /// Returns `true` if [library] is a backend specific library whose members
   /// have special treatment, such as being allowed to extends blacklisted
   /// classes or member being eagerly resolved.
   bool isBackendLibrary(LibraryElement library) {
     // TODO(johnniwinther): Remove this when patching is only done by the
     // JavaScript backend.
     Uri canonicalUri = library.canonicalUri;
     if (canonicalUri == js_backend.BackendHelpers.DART_JS_HELPER ||
         canonicalUri == js_backend.BackendHelpers.DART_INTERCEPTORS) {
@@ skipping 156 matching lines @@
   WorldImpact transformResolutionImpact(ResolutionImpact worldImpact) {
     return worldImpact;
   }
 
   /// Transform the [CodegenImpact] into a [WorldImpact] adding the
   /// backend dependencies for features used in [worldImpact].
   WorldImpact transformCodegenImpact(CodegenImpact worldImpact) {
     return worldImpact;
   }
 }