Replicate mock_sdk.dart in analyzer2dart package.

pkg/analyzer2dart/test/driver_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 48 matching lines @@
     FunctionElement fooElement = functions['foo'];
     expect(world.executableElements.keys, contains(fooElement));
     FunctionDeclaration fooAst = world.executableElements[fooElement];
     expect(fooAst.element, equals(fooElement));
     FunctionElement barElement = functions['bar'];
     expect(
         world.executableElements.keys,
         isNot(contains(functions[barElement])));
   });
 }