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

pkg/front_end/test/fasta/testing.json

 {
 "":"Copyright (c) 2016, 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",
 
   "suites": [
 
     {
@@ skipping 135 matching lines @@
       "../../lib/src/fasta/util",
       "../../tool",
       "../../tool/_fasta",
       "parser/parser_suite.dart",
       "scanner/scanner_suite.dart",
       "compile_test.dart",
       "kompile_test.dart",
       "outline_test.dart",
       "sdk_test.dart",
       "../../../analyzer/lib/src/fasta",
-      "../../../compiler/lib/src/parser",
-      "../../../compiler/lib/src/scanner",
       "../../../../utils/kernel-service/kernel-service.dart",
       "../../../../tools/patch_sdk.dart"
     ],
 
     "exclude": [
       "/third_party/pkg/",
       "/third_party/pkg_tested/",
       "/pkg/analyzer/(?!.*/(fasta|kernel)/)",
-      "/pkg/compiler/lib/src/(?!(scanner|parser)/)",
-      "/pkg/compiler/lib/src/parser/partial_elements\\.dart$",
       "/pkg/js_ast/"
     ]
   }
 }