Replicate mock_sdk.dart in analyzer2dart package.

pkg/analyzer2dart/test/tree_shaker_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.
 
-import 'package:analysis_testing/mock_sdk.dart';
+import 'mock_sdk.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:unittest/unittest.dart';
 
 import '../lib/src/closed_world.dart';
 import '../lib/src/driver.dart';
 
@@ skipping 276 matching lines @@
   /**
    * Asserts that [world] doesn't contain a method with the given qualified
    * name.
    *
    * [qualifiedName] - the qualified name in form 'className.methodName'.
    */
   void assertNoMethod(String qualifiedName) {
     expect(methods, isNot(contains(qualifiedName)));
   }
 }