Change Rectangle.operator== to compare right/bottom instead of width/height.

Change Rectangle.operator== to compare right/bottom instead of width/height.

BUG= http://dartbug.com/16245
R=efortuna@google.com

Committed: https://code.google.com/p/dart/source/detail?r=32155

[Emily Fortuna, 2014-01-29 19:03:51]

why this change though? This just adds another operation (+) to access right and
bottom when width and height were just fields. It's minor, but still.

[Emily Fortuna, 2014-01-29 22:42:20]

lgtm. saw your explanation in the other email.

[Lasse Reichstein Nielsen, 2014-01-30 09:53:39]

Let's just repeat it here, for posterity.

A rectangle represents the points that are between its left/right and top/bottom
edges (inclusive).

Due to doubles losing precision, it's possible to have two rectangles with
different widths but same right edge (ditto height and bottom).

This change makes rectangles equal if they represent the same set of points
(same left/right/top/bottom) and not just if they have the same parameters
(left/width/top/height).

It also addresses the bug report, which had cases where r1.boundingBox(r1) !=
r1. They will now be equal, even if the width of the bounding box isn't exactly
the same as the width of the rectangle.

[Lasse Reichstein Nielsen, 2014-01-30 09:54:52]

Committed patchset #2 manually as r32155 (presubmit successful).