Add hashCode to Object for dart2js

[sethladd]

From issue #3369

[kasperl]

Set owner to @lrhn.

[peter-ahe-google]

I'm stealing this one as I have to remove some dead code from Object.hashCode anyways.

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Set owner to @peter-ahe-google.
Added Started label.

[peter-ahe-google]

Fixed in r12704.

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Added Fixed label.