Fix dart2js warnings and add test to ensure it stays clean.

pkg/front_end/test/fasta/analyze_test.dart

Index: pkg/front_end/test/fasta/analyze_test.dart
diff --git a/pkg/front_end/test/fasta/analyze_test.dart b/pkg/front_end/test/fasta/analyze_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..1088a2afc9ef28043c7b104e6b54546b87207f44
--- /dev/null
+++ b/pkg/front_end/test/fasta/analyze_test.dart
@@ -0,0 +1,15 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:testing/src/run_tests.dart" as testing show main;
+
+main() {
+  // This method is async, but keeps a port open to prevent the VM from exiting
+  // prematurely.
+  return testing.main(<String>[
+    "--config=pkg/front_end/test/fasta/testing.json",
+    "--verbose",
+    "analyze"
+  ]);
+}