Make analyzer strong-mode clean

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 82829eb52bd8782f0d3bc0d60db7a3620e3454ce..ed14805304ff61c38d5f3b0271e94c0674e2973c 100644
--- a/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
+++ b/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
@@ -69,7 +69,7 @@ CompilationUnit check({bool implicitCasts: true, bool implicitDynamic: true}) {
   options.implicitCasts = implicitCasts;
   options.implicitDynamic = implicitDynamic;
   var mockSdk = new MockSdk();
-  mockSdk.context.analysisOptions.strongMode = true;
+  (mockSdk.context.analysisOptions as AnalysisOptionsImpl).strongMode = true;
   context.sourceFactory =
       new SourceFactory([new DartUriResolver(mockSdk), uriResolver]);