Add utility operators/methods to dart:html Point and Rect

[DartBot]

This issue was originally filed by [email protected]

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I would like a way to modify Point and Rect with more operators/methods:
  - get a new Point/Rect x pixels to the left/right/up/down.
  - scale a rectangle in either the x or y direction or both
    - This requires some extra information to determine to what point we're scaling. That is, do we scale and leave top left point where it is? Or keep the center where it is? I think this should be an option.
  - get the center of the rectangle, or the center of the sides
  - get the midpoint between two points
    - for that matter, get the centroid of n points.

Currently Point and Rect and special and individualized, I would love to see a generic Shape or Polygon class with any number of points defining the shape. Then you could get centroids, easily create polygon fills on a canvas, make more interesting bounding boxes for a series of shapes, do rotation, etc.

A Line class would be pretty useful, too, actually. Perhaps a Curve class as well. A Path class, as well, which is a sequence of Curves and Lines or the perimeter of a Shape.

What do you think about any of these ideas?

[blois]

Added this to the Later milestone.

[kevmoo]

Removed Area-HTML label.
Added Area-Library label.

[efortuna]

Removed Priority-Medium label.
Added Priority-Low label.

[kasperl]

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

[kasperl]

Removed Oldschool-Milestone-Later label.