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

pkg/compiler/testing.json

Index: pkg/compiler/testing.json
diff --git a/pkg/compiler/testing.json b/pkg/compiler/testing.json
new file mode 100644
index 0000000000000000000000000000000000000000..0b3ac7547ffbef7a898cb880a6b1153926e4b4d3
--- /dev/null
+++ b/pkg/compiler/testing.json
@@ -0,0 +1,19 @@
+{
+"":"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.md file.",
+
+  "packages": "../../.packages",
+
+  "analyze": {
+
+    "uris": [
+      "tool"
+    ],
+
+    "exclude": [
+      "/third_party/pkg/",
+      "/third_party/pkg_tested/"
+    ]
+  }
+}