[CFE] Example of an added stub or not

This CL does three things:
* It adds a test that shows how a stub is added if it compiles all
  "user libraries" from source, but not when it loads some libraries
  from a dill. This is bad and can lead to leaks (and possibly other
  things).
* It adds the transformation phase to the tests using the link option
  for the linked component. This is needed to avoid a potential crash
  (assert error) in the mixin transformation triggered by the new test
  added above.
* It adds the possibility to create a non-linked compilation in a
  testcase directory with a link.options file by naming the file
  "<whatever>.no_link.dart". This way one can have two expect files
  close together that shows differences between using linked
  dependencies and not without too much duplicated test code.

Change-Id: I170c66fb3e0d4d1263217d64234dc0e88388747d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/156912
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>