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

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 7407ef7a094f4b9a45eb6a160be0965e123c063e..faa4de3e16167b41b6b0d4c23e1eed023f9c085a 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -559,8 +559,18 @@ class JavaScriptBackend extends Backend {
     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);
     });