Fix static typing problem in analysis_server tests.

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 fa683a3a5b28addc4d1e566f1e26f6197699b8af..d1d3358470b729e50fd20b74989cdb8c21a644f7 100644
--- a/pkg/analysis_server/test/mocks.dart
+++ b/pkg/analysis_server/test/mocks.dart
@@ -249,8 +249,8 @@ class MockServerChannel implements ServerCommunicationChannel {
 
   Future<Response> waitForResponse(Request request) {
     String id = request.id;
-    return new Future<Response>(() =>
-        responseController.stream.firstWhere((response) => response.id == id));
+    return new Future<Response>(() => responseController.stream
+        .firstWhere((response) => response.id == id) as Response);
   }
 }