Faster ~/2 for dart2js.
Faster ~/2 for dart2js.

This just adds a special case for 2 to the ~/ operator when the
number being divided is in the unsigned 31-bit range (i.e., where
bit operations work).

There are other possible cases for optimization, but that would
require more elaborate tests with higher overhead.

BUG= http://dartbug.com/14166
R=ngeoffray@google.com

Review URL: https://codereview.chromium.org//27408003

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@29075 260f80e4-7a28-3924-810f-c04153c831b5

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