Remove BackendClasses and JavaScriptBackendClasses.

tests/compiler/dart2js/simple_inferrer_final_field2_test.dart

 // Copyright (c) 2013, 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 a non-used generative constructor does not prevent
 // infering types for fields.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
@@ skipping 20 matching lines @@
         var typesInferrer = compiler.globalInference.typesInferrerInternal;
 
         checkFieldTypeInClass(String className, String fieldName, type) {
           var cls = findElement(compiler, className);
           var element = cls.lookupLocalMember(fieldName);
           Expect.isTrue(
               typesInferrer.getTypeOfElement(element).containsOnly(type));
         }
 
         checkFieldTypeInClass(
-            'A', 'intField', compiler.backend.backendClasses.uint31Class);
+            'A', 'intField', compiler.commonElements.jsUInt31Class);
         checkFieldTypeInClass(
-            'A', 'stringField', compiler.backend.backendClasses.stringClass);
+            'A', 'stringField', compiler.commonElements.jsStringClass);
       }));
 }