Use entities in CommonElements interface.

tests/compiler/dart2js/type_combination_test.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.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/js_backend/backend_helpers.dart';
 import 'package:compiler/src/js_backend/js_backend.dart';
 import 'package:compiler/src/types/types.dart';
 import 'package:compiler/src/world.dart';
@@ skipping 750 matching lines @@
         new TypeUse.instantiation(compiler.commonElements.functionType));
     impactBuilder
         .registerTypeUse(new TypeUse.instantiation(patternImplClass.rawType));
     compiler.enqueuer.resolution.applyImpact(impactBuilder);
     compiler.closeResolution();
     ClosedWorld closedWorld =
         compiler.resolutionWorldBuilder.closedWorldForTesting;
 
     // Grab hold of a supertype for String so we can produce potential
     // string types.
-    patternClass = closedWorld.commonElements.coreLibrary.find('Pattern');
+    LibraryElement coreLibrary = closedWorld.commonElements.coreLibrary;
+    patternClass = coreLibrary.find('Pattern');
 
     nonPrimitive1 = new TypeMask.nonNullSubtype(
         closedWorld.commonElements.mapClass, closedWorld);
     nonPrimitive2 = new TypeMask.nonNullSubtype(
         closedWorld.commonElements.functionClass, closedWorld);
     potentialArray =
         new TypeMask.subtype(closedWorld.commonElements.listClass, closedWorld);
     potentialString = new TypeMask.subtype(patternClass, closedWorld);
     jsInterceptor =
         new TypeMask.nonNullSubclass(helpers.jsInterceptorClass, closedWorld);
@@ skipping 44 matching lines @@
     Expect.notEquals(jsStringOrNull, potentialString,
         "potentialString expected not to be exact JSString");
     Expect.notEquals(jsArrayOrNull, potentialArray,
         "potentialArray expected not to be JSArray subclass");
 
     testUnion(compiler);
     testIntersection(compiler);
     testRegressions(compiler);
   });
 }