dart2js shall work as forgc that allow using "native" keyword

[DartBot]

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

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When compile dart code with dart2js, it complains "Unexpected token" for "native" keyword while "frogc" let go without problem.

I think dart2js shall allow using "native" keyword so dart can access 3rd party JavaScript libraries (such as Google Maps APIs) easily.

[DartBot]

This comment was originally written by ngeoffray@google.com

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Hi Chenhenri,

Thank you for your input. The 'native' keyword is not in the language specification, so we don't plan on supporting it. We are working on a different approach for interacting with 3d party JavaScript libraries, through isolates.

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Added Area-Dart2JS, WontFix labels.