Refactor Library/CompilationUnit and how we define local Scope.

tests/compiler/dart2js/mock_compiler.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('mock_compiler');
 
 #import("dart:uri");
 
 #import("../../../lib/compiler/implementation/elements/elements.dart");
 #import("../../../lib/compiler/implementation/leg.dart");
@@ skipping 172 matching lines @@
   if (found.hasNext()) {
     do {
       print('Additional warning "${found.next()}"');
     } while (found.hasNext());
     fail('Too many warnings');
   }
 }
 
 void importLibrary(LibraryElement target, LibraryElement imported,
                    Compiler compiler) {
-  for (Link<Element> link = imported.topLevelElements; !link.isEmpty();
-       link = link.tail) {
-    compiler.withCurrentElement(link.head, () {
-        target.define(link.head, compiler);
-      });
+  for (var element in imported.localMembers) {
+    compiler.withCurrentElement(element, () {
+      target.addToScope(element, compiler);
+    });
   }
 }
 
 LibraryElement mockLibrary(Compiler compiler, String source) {
   Uri uri = new Uri.fromComponents(scheme: "source");
   var library = new LibraryElement(new Script(uri, new MockFile(source)));
   importLibrary(library, compiler.coreLibrary, compiler);
   return library;
 }