(TBR) simplify check for valid SDK directory

pkg/analysis_server/test/mocks.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 mocks;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/channel.dart';
 import 'package:analysis_server/src/protocol.dart';
 import 'package:unittest/matcher.dart';
 
 /**
- * Answer the path the the SDK relative to the currently running script
- * or throw an exception if it cannot be found.
+ * Answer the absolute path the the SDK relative to the currently running
+ * script or throw an exception if it cannot be found.
  */
 String get sdkPath {
   Uri sdkUri = Platform.script.resolve('../../../sdk/');
-  // Verify that the internal library file exists
-  Uri libFileUri = sdkUri.resolve('lib/_internal/libraries.dart');
-  if (!new File.fromUri(libFileUri).existsSync()) {
-    throw 'Expected Dart SDK at ${sdkUri.path}';
+
+  // Verify the directory exists
+  Directory sdkDir = new Directory.fromUri(sdkUri);
+  if (!sdkDir.existsSync()) {
+    throw 'Specified Dart SDK does not exist: $sdkDir';
   }
-  return sdkUri.path;
+
+  return sdkDir.path;
 }
 
 /**
  * A mock [WebSocket] for testing.
  */
 class MockSocket<T> implements WebSocket {
   StreamController controller = new StreamController();
   MockSocket twin;
   Stream stream;
 
@@ skipping 67 matching lines @@
     responsesReceived.add(response);
     // Wrap send response in future to simulate websocket
     new Future(() => responseController.add(response));
   }
 
   void expectMsgCount({responseCount: 0, notificationCount: 0}) {
     expect(responsesReceived, hasLength(responseCount));
     expect(notificationsReceived, hasLength(notificationCount));
   }
 }