Extract ResolutionWorldBuilderBase from ElementResolutionWorldBuilder

tests/compiler/dart2js/instantiated_classes_test.dart

 // Copyright (c) 2014, 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 instantiated_classes_test;
 
 import 'dart:async';
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
-import 'package:compiler/src/elements/elements.dart' show ClassElement;
+import 'package:compiler/src/elements/entities.dart' show ClassEntity;
 import 'type_test_helper.dart';
 
 void main() {
   asyncTest(() => Future.forEach([
         () => test("class Class {}", ["Class"]),
         () => test(
             """abstract class A {}
                   class Class extends A {}""",
             ["Class"]),
         () => test(
@@ skipping 62 matching lines @@
     [List<String> newClasses = const <String>["Class"]]) {
   StringBuffer mainSource = new StringBuffer();
   mainSource.write('main() {\n');
   for (String newClass in newClasses) {
     mainSource.write('  new $newClass();\n');
   }
   mainSource.write('}');
   return TypeEnvironment
       .create(source, mainSource: mainSource.toString(), useMockCompiler: true)
       .then((env) {
-    Iterable<ClassElement> expectedClasses =
+    Iterable<ClassEntity> expectedClasses =
         directlyInstantiatedClasses.map(env.getElement);
-    Iterable<ClassElement> actualClasses = env
+    Iterable<ClassEntity> actualClasses = env
         .compiler.resolutionWorldBuilder.directlyInstantiatedClasses
         .where((c) => c.library == env.compiler.mainApp);
     Expect.setEquals(expectedClasses, actualClasses);
   });
 }