* avoid // ignore: prefer_const_constructors

* address review comments

Reason: https://github.com/flutter/flutter/issues/4273

MockCanvas to use it instead.

Currently it only supports a very limited set of drawing features, but
adding new ones is easy.

Once the feature set is more complete, and we've used this a bunch, if
it feels right, this API will get promoted to flutter_test.

Answers http://stackoverflow.com/questions/41371449/how-do-you-use-a-textpainter-to-draw-text

Overflow handling works with clipping, adding an ellipsis on the last
line, or fading the last line.

Fixes https://github.com/flutter/flutter/issues/7271

* MultiTapGestureRecognizer previously would assert if there was no
   competition.
 * GestureArenaTeam would always select the first recongizer as the
   winner even if a later recognizer actually accepted the pointer
   sequence.
 * debugPrintStack would fail a type check if maxFrames was non-null.
 * FractionalOffset.lerp would throw a null-pointer exception if its
   second argument was null.

Also, add a number of tests for previously untested lines of code.

The indexes passed to paragraph.getBoxesForRange are based on UTF-16 code
units.  If the caret is at a character that is encoded as multiple code units,
then the start and end indexes should be adjusted accordingly.

Also, make the exception handling for global key listeners slightly more
robust.

This patch improves test coverage for a number of files that had low
coverage. This patch also fixes a few minor bugs found by these tests.

* globalToLocal was just broken when there was a rotation and a
  translation at the same time. This fixes that and adds a test.

* update graphic used by spinning_mixed since the old one went 404.

* simplify some of the code in the demo.

* fix MatrixUtils.transformPoint to be consistent with how we transform
  points elsewhere.

* stop transforming points elsewhere, just use
  MatrixUtils.transformPoint.

* make the Widget binding handle not having a root element.

* make the spinning_mixed demo update its widget tree.

This version of the engine crashes on startup.

This reverts commit a426b6b3.

This feature is the implementation of an accessibility feature that changes the
size of text by a constant factor.

Fixes #5873

The engine now reports coordinates in physical pixels.

Also, fix an old TODO, and add a test of various things on TextStyle.

...requires an engine roll to pick up https://github.com/flutter/engine/pull/3025 and https://github.com/flutter/engine/pull/3027.

These tests should hit some previously untested lines.

Image listeners installed in paint handlers need to know whether the listener
is being called during the paint.

Fixes https://github.com/flutter/flutter/issues/4937

Also, add FractionalOffset.inscribe.

Previously we lerped them backwards.



Fixes #2832

Fixes #1382

Specifically:

* Handle null styles in TextSpan without crashing in toString().

* Handle null children in TextSpan child lists without crashing in
  toString().

* Handle entirely empty TextSpans in toString() explicitly.

* Assert that TextSpans don't contain nulls in various places. This is
  done more often than one might think necessary, because it turns out
  that TextSpan takes a (mutable) List for one of its arguments, so
  who knows what it will contain at any given time. By asserting all
  over the place, hopefully we'll catch it near the change if they do
  change it.

* Add a RichText example to Stocks to exercise RichText and TextSpans.

See also: https://github.com/flutter/flutter/issues/2514, https://github.com/flutter/flutter/issues/2519

We need to apply the alignment to the source rect because the ouput rect fills
the destination rect.

Fixes #1514

We forgot to compare the lengths of the lists.