Fix some warnings.

lib/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('../tree/tree.dart');
 #import('../scanner/scannerlib.dart');
@@ skipping 1286 matching lines @@
       TypeVariableElement variableElement =
           new TypeVariableElement(variableName, element, node);
       TypeVariableType variableType = new TypeVariableType(variableElement);
       variableElement.type = variableType;
       arguments.addLast(variableType);
     }
     return arguments.toLink();
   }
 }
 
-class ClassElement extends ScopeContainerElement
+abstract class ClassElement extends ScopeContainerElement
     implements TypeDeclarationElement {
   final int id;
   InterfaceType type;
   DartType supertype;
   DartType defaultClass;
   Link<DartType> interfaces;
   SourceString nativeName;
   int supertypeLoadState;
   int resolutionState;
 
   // backendMembers are members that have been added by the backend to simplify
   // compilation. They don't have any user-side counter-part.
   Link<Element> backendMembers = const EmptyLink<Element>();
 
   Link<DartType> allSupertypes;
 
   // Lazily applied patch of class members.
   ClassElement patch = null;
   ClassElement origin = null;
 
   ClassElement(SourceString name, Element enclosing, this.id, int initialState)
     : supertypeLoadState = initialState,
       resolutionState = initialState,
       super(name, ElementKind.CLASS, enclosing);
 
+  abstract ClassNode parseNode(Compiler compiler);
+
   InterfaceType computeType(compiler) {
     if (type == null) {
       if (origin === null) {
         ClassNode node = parseNode(compiler);
         Link<DartType> parameters =
             TypeDeclarationElement.createTypeVariables(this,
                                                        node.typeParameters);
         type = new InterfaceType(this, parameters);
       } else {
         type = origin.computeType(compiler);
@@ skipping 621 matching lines @@
 
   MetadataAnnotation ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveMetadataAnnotation(this);
     }
     return this;
   }
 
   String toString() => 'MetadataAnnotation($value, $resolutionState)';
 }