Use async 1.3.0.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1279143004 .

pubspec.yaml

@@ -8,9 +8,7 @@ environment:
 dependencies:
   analyzer: '>=0.23.0 <0.27.0'
   args: '>=0.12.1 <0.14.0'
-
-  # TODO(nweiz): Bring in a hosted version of async when 1.3.0 is released.
-  async: {git: git://github.com/dart-lang/async.git}
+  async: '>=1.3.0 <2.0.0'
   barback: '>=0.14.0 <0.16.0'
   collection: '^1.1.0'
   crypto: '^0.9.0'