Remove warnings from type mirrors and add argument checks.

sdk/lib/_internal/compiler/implementation/mirrors/dart2js_instance_mirrors.dart

Index: sdk/lib/_internal/compiler/implementation/mirrors/dart2js_instance_mirrors.dart
diff --git a/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_instance_mirrors.dart b/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_instance_mirrors.dart
index c3c2b39c3b9032bd4b282b44674fad4987df12e9..e8f5a7013ee9fd2f6a992f6bc3ad437ca438d97e 100644
--- a/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_instance_mirrors.dart
+++ b/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_instance_mirrors.dart
@@ -21,17 +21,12 @@ abstract class ObjectMirrorMixin implements ObjectMirror {
 }
 
 abstract class InstanceMirrorMixin implements InstanceMirror {
-
   bool get hasReflectee => false;
 
   get reflectee {
     throw new UnsupportedError('InstanceMirror.reflectee unsupported.');
   }
 
-  Function operator [](Symbol name) {
-    throw new UnsupportedError('InstanceMirror.operator [] unsupported.');
-  }
-
   delegate(Invocation invocation) {
     throw new UnsupportedError('InstanceMirror.delegate unsupported');
   }
@@ -68,8 +63,7 @@ InstanceMirror _convertConstantToInstanceMirror(
 ////////////////////////////////////////////////////////////////////////////////
 
 class Dart2JsConstantMirror extends Object
-    with ObjectMirrorMixin, InstanceMirrorMixin
-    implements InstanceMirror {
+    with ObjectMirrorMixin, InstanceMirrorMixin {
   final Dart2JsMirrorSystem mirrorSystem;
   final Constant _constant;