Retain elements a finer granularity than library.

dart/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 '../tree/tree.dart';
 import '../util/util.dart';
 import '../resolution/resolution.dart';
@@ skipping 567 matching lines @@
 
 abstract class AmbiguousElement extends Element {
   DualKind get messageKind;
   Map get messageArguments;
   Element get existingElement;
   Element get newElement;
 }
 
 // TODO(kasperl): This probably shouldn't be called an element. It's
 // just an interface shared by classes and libraries.
-abstract class ScopeContainerElement {
+abstract class ScopeContainerElement implements Element {
   Element localLookup(SourceString elementName);
 
   void forEachLocalMember(f(Element element));
 }
 
 abstract class CompilationUnitElement extends Element {
   Script get script;
   PartOf get partTag;
 
   void forEachLocalMember(f(Element element));
@@ skipping 335 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);
 }