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

pkg/compiler/lib/src/js_backend/backend.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.
 
 part of js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 541 matching lines @@
   CoreTypes get coreTypes => compiler.coreTypes;
 
   Resolution get resolution => compiler.resolution;
 
   /// Returns constant environment for the JavaScript interpretation of the
   /// constants.
   JavaScriptConstantCompiler get constants {
     return constantCompilerTask.jsConstantCompiler;
   }
 
-  FunctionElement resolveExternalFunction(FunctionElement element) {
-    if (isForeign(element) || isJsInterop(element)) return element;
+  MethodElement resolveExternalFunction(MethodElement element) {
+    if (isForeign(element)) {
+      return element;
+    }
+    if (isJsInterop(element))  {
+      if (element.memberName == const PublicName('[]') ||
+          element.memberName == const PublicName('[]=')) {
+        reporter.reportErrorMessage(element,
+            MessageKind.JS_INTEROP_INDEX_NOT_SUPPORTED);
+      }
+      return element;
+    }
     return patchResolverTask.measure(() {
       return patchResolverTask.resolveExternalFunction(element);
     });
   }
 
   bool isForeign(Element element) => element.library == helpers.foreignLibrary;
 
   bool isBackendLibrary(LibraryElement library) {
     return library == helpers.interceptorsLibrary ||
            library == helpers.jsHelperLibrary;
@@ skipping 2598 matching lines @@
     }
   }
 
   @override
   void onImpactUsed(ImpactUseCase impactUse) {
     if (impactUse == DeferredLoadTask.IMPACT_USE) {
       resolution.emptyCache();
     }
   }
 }