Improve Future<T>.then<R> inference

pkg/analyzer/test/src/task/strong/strong_test_helper.dart

Index: pkg/analyzer/test/src/task/strong/strong_test_helper.dart
diff --git a/pkg/analyzer/test/src/task/strong/strong_test_helper.dart b/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
index 6e5b83b40ac2be624f5c16d236fe3635768c5cb5..079bda1518f8da3182e65870ea038be7d8062b96 100644
--- a/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
+++ b/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
@@ -66,8 +66,10 @@ CompilationUnit check() {
   AnalysisOptionsImpl options = context.analysisOptions as AnalysisOptionsImpl;
   options.strongMode = true;
   options.strongModeHints = true;
+  var mockSdk = new MockSdk();
+  mockSdk.context.analysisOptions.strongMode = true;
   context.sourceFactory =
-      new SourceFactory([new DartUriResolver(new MockSdk()), uriResolver]);
+      new SourceFactory([new DartUriResolver(mockSdk), uriResolver]);
 
   // Run the checker on /main.dart.
   Source mainSource = uriResolver.resolveAbsolute(new Uri.file('/main.dart'));