dart:geometry library

[DartBot]

This issue was originally filed by @seaneagan

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"dart:html" contains several geometric concepts which would be nice to factor out into a "dart:geometry" or similar:

Point (issue #3053)
Rectangle (issue #3035)
Matrix (issue #3054)

and at least a few other concepts would be useful:

Interval (https://gist.github.com/2719638)
Polygon
Line
Path
Angle
Circle
Ellipse
Square
etc.

[iposva-google]

Added Area-DOM, Triaged labels.

[DartBot]

This comment was originally written by @jinmingjian

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should be.

[iposva-google]

Removed Area-DOM label.
Added Area-HTML label.

[vsmenon]

Added this to the Later milestone.
Removed Type-Defect label.
Added Type-Enhancement label.

[efortuna]

Added html-api label.

[blois]

Removed Area-HTML, html-api labels.
Added Area-Library label.

[floitschG]

cc @efortuna.

[kasperl]

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

[kasperl]

Removed Oldschool-Milestone-Later label.