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

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

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index d2fa5ec96cc6139b48bad64caf247adb783a6ad7..306361621ea9934a2652ce67f9f111d2b02b1fac 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -31,7 +31,8 @@ import '../elements/elements.dart' show
     Element,
     FunctionElement,
     LibraryElement,
-    MetadataAnnotation;
+    MetadataAnnotation,
+    MethodElement;
 import '../enqueue.dart' show
     Enqueuer,
     CodegenEnqueuer,
@@ -282,9 +283,9 @@ abstract class Backend {
     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