Update documentation of Stream.first/last and handleError.
Update documentation of Stream.first/last and handleError.

BUG= http://dartbug.com/11118 http://dartbug.com/11700
R=floitsch@google.com

Review URL: https://codereview.chromium.org//23079015

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@26647 260f80e4-7a28-3924-810f-c04153c831b5

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