Implement tree-shaking based on MirrorsUsed

[peter-ahe-google]

dart2js needs to respect MirrorsUsed metadata in dart:mirrors.

[peter-ahe-google]

Added Library-Mirrors label.

[justinfagnani]

Removed Priority-Medium label.
Added Priority-High, C1 labels.

[peter-ahe-google]

Issue #12125 has been merged into this issue.

[peter-ahe-google]

https://codereview.chromium.org/21242002/

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Added Started label.

[peter-ahe-google]

Marked this as blocking #6490.

[peter-ahe-google]

CL 21242002 landed in r25873.

[peter-ahe-google]

https://codereview.chromium.org/22301009/ reduces the number of symbols stored in compiled output.

[peter-ahe-google]

CL 22301009 landed in r25878.

[larsbak]

Added this to the M7 milestone.

[peter-ahe-google]

I'm lowering the priority: MirrorsUsed is not fully implemented, but can be used to control tree-shaking.

The missing part is supporting metaTargets, which we don't deem a high priority at the moment.

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Removed Priority-High label.
Added Priority-Medium label.

[kasperl]

Removed this from the M7 milestone.
Added this to the M8 milestone.

[kasperl]

Removed this from the M8 milestone.
Added this to the 1.6 milestone.

[kasperl]

Removed this from the 1.6 milestone.
Added Oldschool-Milestone-1.6 label.

[kasperl]

Removed Oldschool-Milestone-1.6 label.