The hello_world app used to measure Dart payload size was broken by a
change in
https://github.com/angular/angular/commit/7ae23adaff2990cf6022af9792c449730d451d1d.
This breakage does not materially affect the size of the generated code,
(before fix: 298819, after fix: 298825), and since it was a runtime
error it was not noticed & not a problem.
Update the app to work again.
Closes#7358
Import dependencies from `angular2/angular2.dart` in `hello_world` to
ensure that any size regressions are caught by our checks.
Update to avoid regressions like #5419.
Closes#5394