gracefully degrade with response to client if fail to access client specified dart sdk

pkg/analysis_server/lib/src/domain_server.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 domain.server;
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/java_io.dart';
@@ skipping 91 matching lines @@
 
     String baseContextId = new DateTime.now().millisecondsSinceEpoch.toRadixString(16);
     String contextId = baseContextId;
     int index = 1;
     while (server.contextMap.containsKey(contextId)) {
       contextId = '$baseContextId-$index';
     }
     AnalysisContext context = AnalysisEngine.instance.createAnalysisContext();
     // TODO(brianwilkerson) Use the information from the request to set the
     // source factory in the context.
+    DirectoryBasedDartSdk sdk;
+    try {
+      sdk = new DirectoryBasedDartSdk(new JavaFile(sdkDirectory));
+    } on Exception catch (e) {
+      // TODO what error code should be returned here?
+      return new Response(request.id, new RequestError(
+          RequestError.CODE_SDK_ERROR, 'Failed to access sdk: $e'));
+    }
     context.sourceFactory = new SourceFactory.con2([
-      new DartUriResolver(new DirectoryBasedDartSdk(new JavaFile(sdkDirectory))),
+      new DartUriResolver(sdk),
       new FileUriResolver(),
       // new PackageUriResolver(),
     ]);
     server.contextMap[contextId] = context;
 
     Response response = new Response(request.id);
     response.setResult(CONTEXT_ID_RESULT, contextId);
     return response;
   }
 
@@ skipping 23 matching lines @@
 
   /**
    * Return the version number of the analysis server.
    */
   Response version(Request request) {
     Response response = new Response(request.id);
     response.setResult(VERSION_RESULT, '0.0.1');
     return response;
   }
 }