Add support for generators

[DartBot]

This issue was originally filed by paulp...@gmail.com

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http://en.wikipedia.org/wiki/Generator_(computer_programming)

This is a great feature, that Ecmascript 6 already has.

[DartBot]

This comment was originally written by paulpmi...@gmail.com

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Sorry, this is duplicate of #­36, the bugtracker had some issues and the issue was created twice.

[sgjesse]

Added Duplicate label.
Marked as being merged into #36.