Minor optimizations to analyzer2dart tree shaker.
Minor optimizations to analyzer2dart tree shaker.

Note that the change from removeAt(0) to removeLast() causes tree
shaking to be in depth-first rather than breadth-first order.  Since
all the elements need to be visited eventually, this seems like it
shouldn't be a problem.

R=johnniwinther@google.com

Review URL: https://codereview.chromium.org//517863003

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@39693 260f80e4-7a28-3924-810f-c04153c831b5

dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk