Update the script that downloads the dart sdk so we can roll it.

tools/dart/update.py

 #!/usr/bin/python
 # Copyright 2015 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.
 
-"""Pulls down the current dart sdk to third_party/dart-sdk/."""
+"""Pulls down the current dart sdk to third_party/dart-sdk/.
+
+You can manually force this to run again by removing
+third_party/dart-sdk/STAMP_FILE, which contains the URL of the SDK that
+was downloaded. Rolling works by updating LINUX_64_SDK to a new URL.
+"""
 
 import os
+import shutil
 import subprocess
 import sys
 
+# How to roll the dart sdk: Just change this url! We write this to the stamp
+# file after we download, and then check the stamp file for differences.
+LINUX_64_SDK = ('http://gsdview.appspot.com/dart-archive/channels/dev/' +
+                'raw/43903/sdk/dartsdk-linux-x64-release.zip')
+
 # Path constants. (All of these should be absolute paths.)
 THIS_DIR = os.path.abspath(os.path.dirname(__file__))
 CHROMIUM_DIR = os.path.abspath(os.path.join(THIS_DIR, '..', '..'))
 DART_SDK_DIR = os.path.join(CHROMIUM_DIR, 'third_party', 'dart-sdk')
-
-# TODO(erg): We might want 32 bit linux too? I don't know of anyone who still
-# uses that though. It looks like clang isn't built 32 bit.
-
-LINUX_64_SDK = ('http://gsdview.appspot.com/dart-archive/channels/dev/' +
-                'raw/43808/sdk/dartsdk-linux-x64-release.zip')
-
 OUTPUT_FILE = os.path.join(DART_SDK_DIR, 'dartsdk-linux-x64-release.zip')
+STAMP_FILE = os.path.join(DART_SDK_DIR, 'STAMP_FILE')
 
 def RunCommand(command, fail_hard=True):
   """Run command and return success (True) or failure; or if fail_hard is
      True, exit on failure."""
 
   print 'Running %s' % (str(command))
   if subprocess.call(command, shell=False) == 0:
     return True
   print 'Failed.'
   if fail_hard:
     sys.exit(1)
   return False
 
 def main():
-  # For version one, we don't actually redownload if the sdk is already
-  # present. This will be replaced with download_from_google_storage once
-  # it supports tarballs.
-  #
-  # You can explicitly redownload this by blowing away your
-  # third_party/dart-sdk/ directory.
-  if not os.path.exists(OUTPUT_FILE):
+  # Only get the SDK if we don't have a stamp for or have an out of date stamp
+  # file.
+  get_sdk = False
+  if not os.path.exists(STAMP_FILE):
+    get_sdk = True
+  else:
+    # Get the contents of the stamp file.
+    with open(STAMP_FILE, "r") as stamp_file:
+      stamp_url = stamp_file.read().replace('\n', '')
+      if stamp_url != LINUX_64_SDK:
+        get_sdk = True
+
+  if get_sdk:
+    # Completely remove all traces of the previous SDK.
+    shutil.rmtree(DART_SDK_DIR)
+    os.mkdir(DART_SDK_DIR)
+
     wget_command = ['wget', '-N', '-c', LINUX_64_SDK, '-P', DART_SDK_DIR]
     if not RunCommand(wget_command, fail_hard=False):
       print "Failed to get dart sdk from server."
       return
 
-    unzip_command = ['unzip', '-q', OUTPUT_FILE, '-d', DART_SDK_DIR]
+    unzip_command = ['unzip', '-o', '-q', OUTPUT_FILE, '-d', DART_SDK_DIR]
     if not RunCommand(unzip_command, fail_hard=False):
       print "Failed to unzip the dart sdk."
 
+    # Write our stamp file so we don't redownload the sdk.
+    with open(STAMP_FILE, "w") as stamp_file:
+      stamp_file.write(LINUX_64_SDK)
+
 if __name__ == '__main__':
   sys.exit(main())