Dart: Change url of Dart repository in gclient recipe module Dart config.

Bug:
Change-Id: I031f5787a87a4bddbd3492912814476daf2c9b4f
Reviewed-on: https://chromium-review.googlesource.com/738118
Commit-Queue: William Hesse <whesse@google.com>
Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>

recipes/recipe_modules/gclient/config.py

@@ -269,8 +269,7 @@ def boringssl(c):
 def dart(c):
   soln = c.solutions.add()
   soln.name = 'sdk'
-  soln.url = ('https://chromium.googlesource.com/external/github.com/' +
-              'dart-lang/sdk.git')
+  soln.url = ('https://dart.googlesource.com/sdk.git')
   soln.deps_file = 'DEPS'
   soln.managed = False