Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/world_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.
 
 library world_test;
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'type_test_helper.dart';
 import 'package:compiler/src/common/names.dart';
@@ skipping 225 matching lines @@
         new F2();
         new G2();
         new G3();
         new H4();
       }
       """,
       useMockCompiler: false);
   ClosedWorld closedWorld = env.closedWorld;
 
   check(String name, {bool hasStrictSubtype, bool hasOnlySubclasses}) {
-    ClassEntity cls = env.getElement(name);
+    ClassElement cls = env.getElement(name);
     Expect.equals(hasStrictSubtype, closedWorld.hasAnyStrictSubtype(cls),
         "Unexpected hasAnyStrictSubtype property on $cls.");
     Expect.equals(hasOnlySubclasses, closedWorld.hasOnlySubclasses(cls),
         "Unexpected hasOnlySubclasses property on $cls.");
   }
 
   check("Object", hasStrictSubtype: true, hasOnlySubclasses: true);
 
   // No instantiated Ax classes.
   check("A", hasStrictSubtype: false, hasOnlySubclasses: true);
@@ skipping 277 matching lines @@
         new F();
         new G();
         new H();
         new I();
         new J();
       }
       """,
       useMockCompiler: false);
   ClosedWorld closedWorld = env.closedWorld;
 
-  ClassElement Object_ = env.getElement("Object");
   ClassElement A = env.getElement("A");
   ClassElement B = env.getElement("B");
   ClassElement C = env.getElement("C");
   ClassElement D = env.getElement("D");
   ClassElement F = env.getElement("F");
   ClassElement G = env.getElement("G");
   ClassElement H = env.getElement("H");
   ClassElement I = env.getElement("I");
   ClassElement J = env.getElement("J");
 
@@ skipping 45 matching lines @@
   check(B, ClassQuery.EXACT, C, ClassQuery.EXACT, []);
   check(B, ClassQuery.EXACT, C, ClassQuery.SUBCLASS, []);
   check(B, ClassQuery.SUBCLASS, C, ClassQuery.EXACT, []);
   check(B, ClassQuery.EXACT, C, ClassQuery.SUBTYPE, []);
   check(B, ClassQuery.SUBTYPE, C, ClassQuery.EXACT, []);
   check(B, ClassQuery.SUBCLASS, C, ClassQuery.SUBCLASS, []);
   check(B, ClassQuery.SUBCLASS, C, ClassQuery.SUBTYPE, []);
   check(B, ClassQuery.SUBTYPE, C, ClassQuery.SUBCLASS, [G]);
   check(B, ClassQuery.SUBTYPE, C, ClassQuery.SUBTYPE, [F, G]);
 }