Closes#10503
It is possible for code in `beforeEach` to capture and fork a zone
(for example creating `NgZone` in `beforeEach`). Subsequently the code
in `it` may chose to do `fakeAsync`. The issue is that because the
code in `it` can use `NgZone` from the `beforeEach`. it effectively can
escape the `fakeAsync` zone. A solution is to run all of the test in
`ProxyZone` which allows a test to dynamically replace the rules at any
time. This allows the `beforeEach` to fork a zone, and then `it` to
retroactively became `fakeAsync` zone.
Miško Hevery
Alex Eagle
Tobias Bosch