Updated invoke_call_through* tests to make them strong mode clean, which
mostly involved changing var -> dynamic in some places.
Bug:
Change-Id: I3ac946bdfadd09d6459c7f104068be4cb0c49c1d
Reviewed-on: https://dart-review.googlesource.com/3781
Commit-Queue: Ben Konyi <bkonyi@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>