Cache the context proxy. Don't use window for global 'this'.
Cache the context proxy. Don't use window for global 'this'.

BUG=
R=ahe@google.com, alexandre.ardhuin@gmail.com, vsm@google.com

Review URL: https://codereview.chromium.org//25138003

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@28181 260f80e4-7a28-3924-810f-c04153c831b5

dart-archive/browser