perf: use faster check for $$ prefix

Use two calls to charAt instead of substr to detect a $$prefix in the shallowCopy functions.
This makes shallowCopy 25-50% faster (depending on which browser is used).
http://jsperf.com/angular-shallow-copy

Closes #5457
Merge branch 'master' of https://github.com/angular/angular.js
componentizd!