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

pkg/analysis_server/lib/src/domain_server.dart

Index: pkg/analysis_server/lib/src/domain_server.dart
diff --git a/pkg/analysis_server/lib/src/domain_server.dart b/pkg/analysis_server/lib/src/domain_server.dart
index 1e37f24a69a38d6fdacc023419823dcae5cafa84..3924d1ad31a9fb9e16c37d21b4ee33d891b923b2 100644
--- a/pkg/analysis_server/lib/src/domain_server.dart
+++ b/pkg/analysis_server/lib/src/domain_server.dart
@@ -109,8 +109,16 @@ class ServerDomainHandler implements RequestHandler {
     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(),
     ]);