perf(view cache): Avoid http.get

This change makes templateUrl component creation 100% faster.

It is a stop-gap until #1107 is fixed.

Closes #1108
feat(element binder): Use a child scope instead of Scope.watch(context:o)

This is the only place where Scope.watch(context:o) is used and it is
in the way of optimizations.

For #1086
feat(scope): Deprecate Scope.watch's context parameter.

Scope.watch should no longer switch context.  Instead of saying

    scope.watch(expression, (v, _) { }, context: o);

Create a new isolate child scope

    scope.createChild(o).watch(expression, (v, _) { });