Gracefully degrade if no source for completion

pkg/analysis_server/lib/src/protocol.dart

Index: pkg/analysis_server/lib/src/protocol.dart
diff --git a/pkg/analysis_server/lib/src/protocol.dart b/pkg/analysis_server/lib/src/protocol.dart
index 31e52c9c88c9039625bdb27496b9b2527a1c933d..1377bc342513566fb101f0c5d112a40095af81b2 100644
--- a/pkg/analysis_server/lib/src/protocol.dart
+++ b/pkg/analysis_server/lib/src/protocol.dart
@@ -826,6 +826,14 @@ class Response {
           error: new RequestError(
               RequestErrorCode.UNKNOWN_REQUEST, 'Unknown request'));
 
+  /**
+   * Initialize a newly created instance to represent an error condition caused
+   * by a [request] referencing a source that does not exist.
+   */
+  Response.unknownSource(Request request) : this(request.id,
+          error: new RequestError(
+              RequestErrorCode.UNKNOWN_SOURCE, 'Unknown source'));
+
   Response.unsupportedFeature(String requestId, String message) : this(
           requestId,
           error: new RequestError(