Fix type check to perform a subtype test at top level instead of an

Fix type check to perform a subtype test at top level instead of an
assignability test. This only makes a difference when assigning an instance of
a function type.
Fix a bad language test.
Disable a bad co19 test (issue 99 filed).
Change error message for dynamic type error.
Committed: https://code.google.com/p/dart/source/detail?r=5413

[srdjan, 2012-03-12 18:40:02]

LGTM