Fix some warnings.

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.
 
 abstract class TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   DartType getType(TypeAnnotation annotation);
   bool isParameterChecked(Element element);
 }
@@ skipping 2719 matching lines @@
                                           MessageKind.CANNOT_RESOLVE, [name]);
     } else if (e.kind === ElementKind.TYPEDEF) {
       error(node, MessageKind.CANNOT_INSTANTIATE_TYPEDEF, [name]);
     } else if (e.kind !== ElementKind.CLASS && e.kind !== ElementKind.PREFIX) {
       error(node, MessageKind.NOT_A_TYPE, [name]);
     }
     return e;
   }
 }
 
-class Scope {
+abstract class Scope {
   final Element element;
   final Scope parent;
 
   Scope(this.parent, this.element);
   abstract Element add(Element element);
 
   Element lookup(SourceString name) {
     Element result = localLookup(name);
     if (result != null) return result;
     return parent.lookup(name);
@@ skipping 128 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';
 }