Use entities in CommonElements interface.

tests/compiler/dart2js/minimal_resolution_test.dart

 // Copyright (c) 2015, 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.
 
 // Test that elements are not needlessly required by dart2js.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/enqueue.dart';
@@ skipping 26 matching lines @@
     {bool proxyConstantComputed: false, bool deprecatedClass: false}) async {
   CompilationResult result = await runCompiler(
       memorySourceFiles: {'main.dart': code}, options: ['--analyze-only']);
   Expect.isTrue(result.isSuccess);
   Compiler compiler = result.compiler;
   Expect.equals(
       proxyConstantComputed,
       compiler.resolution.wasProxyConstantComputedTestingOnly,
       "Unexpected computation of proxy constant.");
 
+  LibraryElement coreLibrary = compiler.commonElements.coreLibrary;
   checkInstantiated(
-      compiler,
-      compiler.commonElements.coreLibrary.find('_Proxy'),
-      proxyConstantComputed);
-  checkInstantiated(compiler,
-      compiler.commonElements.coreLibrary.find('Deprecated'), deprecatedClass);
+      compiler, coreLibrary.find('_Proxy'), proxyConstantComputed);
+  checkInstantiated(compiler, coreLibrary.find('Deprecated'), deprecatedClass);
 
   LibraryElement jsHelperLibrary =
       compiler.libraryLoader.lookupLibrary(BackendHelpers.DART_JS_HELPER);
   jsHelperLibrary.forEachLocalMember((Element element) {
     Uri uri = element.compilationUnit.script.resourceUri;
     if (element.isClass && uri.path.endsWith('annotations.dart')) {
       checkInstantiated(compiler, element, false);
     }
   });
 }