Add a test to test that substitutions are emitted for classes that are only used as type arguments.

R=floitsch@google.com

Review URL: https://codereview.chromium.org//771673002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42059 260f80e4-7a28-3924-810f-c04153c831b5
Remove js.ArrayElement.
Instead introduce js.ArrayHole to mark empty spots in Arrays.

BUG=
R=floitsch@google.com

Review URL: https://codereview.chromium.org//750323003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42060 260f80e4-7a28-3924-810f-c04153c831b5