Typecheck some of ui/webui/resources/js/ with Closure compiler.

third_party/closure_compiler/checker.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from collections import defaultdict
 import os
 import re
 import subprocess
 import sys
 import tempfile
@@ skipping 70 matching lines @@
     "--jscomp_error=misplacedTypeAnnotation",
     "--jscomp_error=missingProperties",
     "--jscomp_error=missingReturn",
     "--jscomp_error=nonStandardJsDocs",
     "--jscomp_error=suspiciousCode",
     "--jscomp_error=undefinedNames",
     "--jscomp_error=undefinedVars",
     "--jscomp_error=unknownDefines",
     "--jscomp_error=uselessCode",
     "--jscomp_error=visibility",
+    # TODO(dbeam): happens when the same file is <include>d multiple times.
+    "--jscomp_off=duplicate",
   ]
 
   _found_java = False
 
   _jar_command = [
     "java",
     "-jar",
     "-Xms1024m",
     "-server",
     "-XX:+TieredCompilation"
@@ skipping 107 matching lines @@
 
     output = self._format_errors(map(self._fix_up_error, errors))
     if runner_cmd.returncode:
       self._fatal("Error in: " + file + ("\n" + output if output else ""))
     elif output:
       self._debug("Output: " + output)
    
     self._clean_up()
 
     return runner_cmd.returncode == 0