Fixes https://github.com/flutter/flutter/issues/6774

This name is consistent with how the other `dart:` libraries are named now.

* Remove the Point class.

* Add Size.contains and Rect.translate for consistency

* Make Offset and Size compare == with subclasses.

Without this we would break the _DebugSize hack.

* Fix copy/pasta

* Fix the examples in the engine repo

Fixes https://github.com/flutter/flutter/issues/8809

* link from Color to Colors

* from review

See https://github.com/flutter/flutter/issues/6807

See https://github.com/flutter/flutter/issues/7431

Fixes https://github.com/flutter/flutter/issues/7200

The Skia team says these APIs are outdated and should be removed. They
do not appear to have any clients.

Fixes https://github.com/flutter/flutter/issues/7202
Fixes https://github.com/flutter/flutter/issues/7204

We no longer offer mojo handles via this mechanism.

Sometimes it is convenient to construct a color from an opacity rather than an
alpha value.

Fixes https://github.com/flutter/flutter/issues/5205

Apparently Skia would like to remove these enum values.

Fixes https://github.com/flutter/flutter/issues/5912

* Added drawArc to Canvas.

Added a way to call SkCanvas::drawArc from Dart. This method is
optimized for the arc case and should be faster that drawing paths.

* Removed implementation details from docs.

* Added more details to the docs.

Fixes #69

This patch moves the remainder of painting and compositing into their
new home (and updates their style to match Google C++ Style).

This should make almost every Canvas call significantly faster.

These parts don't depend on engine anymore. Also reformat them to match Google
C++ style.

Because we're not using array buffers to marshall the vertex data for
drawVertices and drawAtlas, we need to process the paint argument first so that
we don't call Dart_GetField after creating a view into an array buffer.

* Implement Canvas.drawPoints

This enables drawing polygons faster than using Paths.

Fixes https://github.com/flutter/flutter/issues/92

* Implement more Path functions

Path.fillType
Path.addPolygon
Path.addPath
Path.extendWithPath
Path.transform

Fixes https://github.com/flutter/flutter/issues/252

The only remaining calls to Dart_GetField are for Paint, which will be
handled in a later patch.

This patch removes a number of the classes that were generating
Dart_GetField calls, particularly related to drawAtlas and drawVertices.
The functions that used these classes now use primatives instead, which
is more efficient.

This patch removes spurious dependences on other parts of the engine.

This is the only readback from the canvas API. Having a write-only interface
will unlock future optimizations. This function has no clients.

Fixes #4254

Fix comments from https://github.com/flutter/engine/pull/2721

I gave up before I was completely done.

Items still remaining include:

 * drawVertices
 * drawAtlas
 * TransferMode.*
 * VertexMode

Also:
 * Add a convenience constructor to RSTransform
 * Add some convenience methods to Rect and Size
 * Make some defaults more explicit

These Dart_GetField calls show up on traces. This patch removes almost all of
them from the Canvas interface. We still have a few in drawAltas and
drawVertices, which will be removed in a later patch.

Particularly, add support for blur filters to implement backdrop blur effects.