Change interfaces to abstract classes in dart2js compiler.

lib/compiler/implementation/resolver.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.
 
-interface TreeElements {
+abstract class TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   DartType getType(TypeAnnotation annotation);
   bool isParameterChecked(Element element);
 }
 
 class TreeElementMapping implements TreeElements {
   final Map<Node, Element> map;
   final Map<Node, Selector> selectors;
   final Map<TypeAnnotation, DartType> types;
@@ skipping 694 matching lines @@
 
   void internalError(Node node, String message) {
     compiler.internalError(message, node: node);
   }
 
   void unimplemented(Node node, String message) {
     compiler.unimplemented(message, node: node);
   }
 }
 
-interface LabelScope {
+abstract class LabelScope {
   LabelScope get outer;
   LabelElement lookup(String label);
 }
 
 class LabeledStatementLabelScope implements LabelScope {
   final LabelScope outer;
   final Map<String, LabelElement> labels;
   LabeledStatementLabelScope(this.outer, this.labels);
   LabelElement lookup(String labelName) {
     LabelElement label = labels[labelName];
@@ skipping 2029 matching lines @@
 
   Element localLookup(SourceString name) => library.find(name);
   Element lookup(SourceString name) => localLookup(name);
   Element lexicalLookup(SourceString name) => localLookup(name);
 
   Element add(Element newElement) {
     throw "Cannot add an element in the top scope";
   }
   String toString() => '$element';
 }