Use the *correct* type for the fix to analysis_server/test/mocks.dart.

pkg/analysis_server/test/mocks.dart

Index: pkg/analysis_server/test/mocks.dart
diff --git a/pkg/analysis_server/test/mocks.dart b/pkg/analysis_server/test/mocks.dart
index d1d3358470b729e50fd20b74989cdb8c21a644f7..5e8e954ea71c29613745da4ea25370ad11ddaca6 100644
--- a/pkg/analysis_server/test/mocks.dart
+++ b/pkg/analysis_server/test/mocks.dart
@@ -250,7 +250,7 @@ class MockServerChannel implements ServerCommunicationChannel {
   Future<Response> waitForResponse(Request request) {
     String id = request.id;
     return new Future<Response>(() => responseController.stream
-        .firstWhere((response) => response.id == id) as Response);
+        .firstWhere((response) => response.id == id) as Future<Response>);
   }
 }