Remove support for interface in dart2js.

sdk/lib/_internal/compiler/implementation/elements/elements.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 elements;
 
 import 'dart:uri';
 
 import 'modelx.dart';
 import '../tree/tree.dart';
@@ skipping 751 matching lines @@
   void set thisType(InterfaceType value);
   void set supertype(DartType value);
   void set allSupertypes(Link<DartType> value);
   void set interfaces(Link<DartType> value);
   void set patch(ClassElement value);
   void set origin(ClassElement value);
   void set supertypeLoadState(int value);
   void set resolutionState(int value);
   void set nativeTagInfo(SourceString value);
 
-  // TODO(kasperl): These seem outdated.
-  bool isInterface();
-  DartType get defaultClass;
-  void set defaultClass(DartType value);
-
   bool isObject(Compiler compiler);
   bool isSubclassOf(ClassElement cls);
   bool implementsInterface(ClassElement intrface);
   bool isShadowedByField(Element fieldMember);
 
   ClassElement ensureResolved(Compiler compiler);
 
   void addMember(Element element, DiagnosticListener listener);
   void addToScope(Element element, DiagnosticListener listener);
 
@@ skipping 90 matching lines @@
   int get resolutionState;
   Token get beginToken;
   Token get endToken;
 
   // TODO(kasperl): Try to get rid of these.
   void set annotatedElement(Element value);
   void set resolutionState(int value);
 
   MetadataAnnotation ensureResolved(Compiler compiler);
 }