'Sort unit/class members' refactoring implementation.

pkg/analysis_server/test/mock_sdk.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 testing.mock_sdk;
 
 import 'package:analyzer/file_system/file_system.dart' as resource;
 import 'package:analyzer/file_system/memory_file_system.dart' as resource;
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/sdk.dart';
@@ skipping 196 matching lines @@
 
     String path = uriToPath[dartUri];
     if (path != null) {
       resource.File file = provider.getResource(path);
       Uri uri = new Uri(scheme: 'dart', path: dartUri.substring(5));
       return file.createSource(uri);
     }
 
     // If we reach here then we tried to use a dartUri that's not in the
     // table above.
-    throw unimplemented;
+    return null;
   }
 }
 
 
 class _MockSdkLibrary implements SdkLibrary {
   final String shortName;
   final String path;
   final String content;
 
   const _MockSdkLibrary(this.shortName, this.path, this.content);
@@ skipping 14 matching lines @@
   bool get isInternal => throw unimplemented;
 
   @override
   bool get isShared => throw unimplemented;
 
   @override
   bool get isVmLibrary => throw unimplemented;
 
   UnimplementedError get unimplemented => new UnimplementedError();
 }