Add pretty-printing to unittest and more thoroughly print values.

This adds a prettyPrint function that knows to respect line lengths and avoid
printing too much of very large objects. Because that function has built-in
guards against over-printing, the default error formatter is changed to print
the actual value by default.

This also tweaks the output of some matchers to take advantage of the assumption
that the full actual value will always be printed.

R=gram@google.com

Review URL: https://codereview.chromium.org//14600029

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/unittest@22873 260f80e4-7a28-3924-810f-c04153c831b5