This also covers "closurization" of the .call method.
This relies on the fact that f.call === f if f is a closure.
This means that adding a simple getter to _FunctionImpl that
returns this is enough. No need for a separate dispatch.
This CL reverts the previous fix for issue 12602 (modulo the
additional tests).
BUG=dartbug.com/17473, dartbug.com/12602
R=iposva@google.com
Review URL: https://codereview.chromium.org//200193002
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@33736 260f80e4-7a28-3924-810f-c04153c831b5