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

tests/compiler/dart2js/analyze_test.dart

Index: tests/compiler/dart2js/analyze_test.dart
diff --git a/tests/compiler/dart2js/analyze_test.dart b/tests/compiler/dart2js/analyze_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..27e50dd42f151378ca3abd02109bedcc094af0ae
--- /dev/null
+++ b/tests/compiler/dart2js/analyze_test.dart
@@ -0,0 +1,12 @@
+// 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/compiler/testing.json", "--verbose", "analyze"]);
+}