Add support for generators

[DartBot]

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

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Harmony's implementation of generators: http://wiki.ecmascript.org/doku.php?id=harmony:generators

[DartBot]

This comment was originally written by drfibonacci@google.com

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Removed Type-Defect label.
Added Type-Enhancement, Area-Language, Triaged labels.

[DartBot]

This comment was originally written by carstenkl...@yahoo.de

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duplicate of #­36 ?

[floitschG]

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