Retain elements a finer granularity than library.

dart/tests/compiler/dart2js/mock_compiler.dart

Index: dart/tests/compiler/dart2js/mock_compiler.dart
diff --git a/dart/tests/compiler/dart2js/mock_compiler.dart b/dart/tests/compiler/dart2js/mock_compiler.dart
index 950a7e5ebbc2cb3eebb519ac4a7847a7ff739cd5..ec113a8a5aa486ad0d954688b8a14b1359c58cde 100644
--- a/dart/tests/compiler/dart2js/mock_compiler.dart
+++ b/dart/tests/compiler/dart2js/mock_compiler.dart
@@ -340,7 +340,7 @@ class MockCompiler extends Compiler {
     errors = [];
   }
 
-  TreeElementMapping resolveStatement(String text) {
+  CollectingTreeElements resolveStatement(String text) {
     parsedTree = parseStatement(text);
     return resolveNodeStatement(parsedTree, mainApp);
   }
@@ -380,10 +380,6 @@ class MockCompiler extends Compiler {
     // Do nothing. The mock core library is already handled in the constructor.
   }
 
-  void importCoreLibrary(LibraryElement library) {
-    scanner.importLibrary(library, coreLibrary, null);
-  }
-
   Uri translateResolvedUri(LibraryElement importingLibrary,
                            Uri resolvedUri, Node node) => resolvedUri;
 
@@ -427,7 +423,7 @@ void compareWarningKinds(String text, expectedWarnings, foundWarnings) {
   }
 }
 
-void importLibrary(LibraryElement target, LibraryElement imported,
+void importLibrary(LibraryElement target, LibraryElementX imported,
                    Compiler compiler) {
   for (var element in imported.localMembers) {
     compiler.withCurrentElement(element, () {