ES6 Style: add presubmit prompt about => in code that might run on iOS9

PRESUBMIT.py

 # Copyright (c) 2012 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.
 
 """Top-level presubmit script for Chromium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into depot_tools.
 """
 
@@ skipping 1907 matching lines @@
 
   if files:
     return [output_api.PresubmitError(
         'Found base::Singleton<T> in the following header files.\n' +
         'Please move them to an appropriate source file so that the ' +
         'template gets instantiated in a single compilation unit.',
         files) ]
   return []
 
 
-def _CheckNoDeprecatedCompiledResourcesGYP(input_api, output_api):
+def _CheckNoDeprecatedCompiledResourcesGyp(input_api, output_api):
   """Checks for old style compiled_resources.gyp files."""
   is_compiled_resource = lambda fp: fp.endswith('compiled_resources.gyp')
 
   added_compiled_resources = filter(is_compiled_resource, [
     f.LocalPath() for f in input_api.AffectedFiles() if f.Action() == 'A'
   ])
 
   if not added_compiled_resources:
     return []
 
@@ skipping 23 matching lines @@
 
   # Functions
   ( "-webkit-gradient", "gradient" ),
   ( "-webkit-repeating-gradient", "repeating-gradient" ),
   ( "-webkit-linear-gradient", "linear-gradient" ),
   ( "-webkit-repeating-linear-gradient", "repeating-linear-gradient" ),
   ( "-webkit-radial-gradient", "radial-gradient" ),
   ( "-webkit-repeating-radial-gradient", "repeating-radial-gradient" ),
 ]
 
-def _CheckNoDeprecatedCSS(input_api, output_api):
+def _CheckNoDeprecatedCss(input_api, output_api):
   """ Make sure that we don't use deprecated CSS
       properties, functions or values. Our external
       documentation and iOS CSS for dom distiller
       (reader mode) are ignored by the hooks as it
       needs to be consumed by WebKit. """
   results = []
   file_inclusion_pattern = (r".+\.css$",)
   black_list = (_EXCLUDED_PATHS +
                 _TEST_CODE_EXCLUDED_PATHS +
                 input_api.DEFAULT_BLACK_LIST +
@@ skipping 14 matching lines @@
               (fpath.LocalPath(), line_num, deprecated_value, value)))
   return results
 
 
 _DEPRECATED_JS = [
   ( "__lookupGetter__", "Object.getOwnPropertyDescriptor" ),
   ( "__defineGetter__", "Object.defineProperty" ),
   ( "__defineSetter__", "Object.defineProperty" ),
 ]
 
-def _CheckNoDeprecatedJS(input_api, output_api):
+def _CheckNoDeprecatedJs(input_api, output_api):
   """Make sure that we don't use deprecated JS in Chrome code."""
   results = []
   file_inclusion_pattern = (r".+\.js$",)  # TODO(dbeam): .html?
   black_list = (_EXCLUDED_PATHS + _TEST_CODE_EXCLUDED_PATHS +
                 input_api.DEFAULT_BLACK_LIST)
   file_filter = lambda f: input_api.FilterSourceFile(
       f, white_list=file_inclusion_pattern, black_list=black_list)
   for fpath in input_api.AffectedFiles(file_filter=file_filter):
     for lnum, line in fpath.ChangedContents():
       for (deprecated, replacement) in _DEPRECATED_JS:
         if deprecated in line:
           results.append(output_api.PresubmitError(
               "%s:%d: Use of deprecated JS %s, use %s instead" %
               (fpath.LocalPath(), lnum, deprecated, replacement)))
   return results
 
 
+def _CheckForRiskyJsFeatures(input_api, output_api):
+  maybe_ios_js = (r"^(ios|components|ui\/webui\/resources)\/.+\.js$", )
+  file_filter = lambda f: input_api.FilterSourceFile(f, white_list=maybe_ios_js)
+
+  arrow_lines = []
+  for f in input_api.AffectedFiles(file_filter=file_filter):
+    for lnum, line in f.ChangedContents():
+      if ' => ' in line:
+        arrow_lines.append((f.LocalPath(), lnum))
+
+  if not arrow_lines:
+    return []
+
+  return [output_api.PresubmitPromptWarning("""
+Use of => operator detected in:
+%s
+Please ensure your code does not run on iOS9 (=> (arrow) does not work there).
+https://chromium.googlesource.com/chromium/src/+/master/docs/es6_chromium.md#Arrow-Functions
+""" % "\n".join("  %s:%d\n" % line for line in arrow_lines))]
+
+
 def _AndroidSpecificOnUploadChecks(input_api, output_api):
   """Groups checks that target android code."""
   results = []
   results.extend(_CheckAndroidCrLogUsage(input_api, output_api))
   results.extend(_CheckAndroidNewMdpiAssetLocation(input_api, output_api))
   results.extend(_CheckAndroidToastUsage(input_api, output_api))
   return results
 
 
 def _CommonChecks(input_api, output_api):
@@ skipping 28 matching lines @@
   results.extend(_CheckAddedDepsHaveTargetApprovals(input_api, output_api))
   results.extend(
       input_api.canned_checks.CheckChangeHasNoTabs(
           input_api,
           output_api,
           source_file_filter=lambda x: x.LocalPath().endswith('.grd')))
   results.extend(_CheckSpamLogging(input_api, output_api))
   results.extend(_CheckForAnonymousVariables(input_api, output_api))
   results.extend(_CheckCygwinShell(input_api, output_api))
   results.extend(_CheckUserActionUpdate(input_api, output_api))
-  results.extend(_CheckNoDeprecatedCSS(input_api, output_api))
-  results.extend(_CheckNoDeprecatedJS(input_api, output_api))
+  results.extend(_CheckNoDeprecatedCss(input_api, output_api))
+  results.extend(_CheckNoDeprecatedJs(input_api, output_api))
   results.extend(_CheckParseErrors(input_api, output_api))
   results.extend(_CheckForIPCRules(input_api, output_api))
   results.extend(_CheckForWindowsLineEndings(input_api, output_api))
   results.extend(_CheckSingletonInHeaders(input_api, output_api))
-  results.extend(_CheckNoDeprecatedCompiledResourcesGYP(input_api, output_api))
+  results.extend(_CheckNoDeprecatedCompiledResourcesGyp(input_api, output_api))
   results.extend(_CheckPydepsNeedsUpdating(input_api, output_api))
   results.extend(_CheckJavaStyle(input_api, output_api))
   results.extend(_CheckIpcOwners(input_api, output_api))
   results.extend(_CheckMojoUsesNewWrapperTypes(input_api, output_api))
   results.extend(_CheckUselessForwardDeclarations(input_api, output_api))
+  results.extend(_CheckForRiskyJsFeatures(input_api, output_api))
 
   if any('PRESUBMIT.py' == f.LocalPath() for f in input_api.AffectedFiles()):
     results.extend(input_api.canned_checks.RunUnitTestsInDirectory(
         input_api, output_api,
         input_api.PresubmitLocalPath(),
         whitelist=[r'^PRESUBMIT_test\.py$']))
   return results
 
 
 def _CheckPatchFiles(input_api, output_api):
@@ skipping 233 matching lines @@
   results.extend(input_api.canned_checks.CheckTreeIsOpen(
       input_api,
       output_api,
       json_url='http://chromium-status.appspot.com/current?format=json'))
 
   results.extend(input_api.canned_checks.CheckChangeHasBugField(
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   return results