This reverts commit d67f47b2.

Reland "Text inline widgets, TextSpan rework (#30069)" with improved backwards compatibility (#34051)

This reverts commit 14414f35.

This reverts commit 86862c1e.

* Merge in changes for inline widgets

* Make analyzer happy, fix tests

* Fix analyzer

* Add missing doc

* Docs on RichText

* Fix analyzer some more

* Remove whitespace at end of line

* Update goldens

* Text fixes

* Analyzer

* Disallow cursor from appearing beyond the width of the input.

* Test that verifies the cursor can't exceed the width of the input

* Use constant from editable.dart to explain 1 pixel difference in test

* Fix failing test that tested the case of overflowing spaces

* Fix bug in calculation of downstream text distance when aligned: right

* After having affinity explained to me, match getOffset and tests to
expected behavior

* Clean up test formatting and fix analyze errors

* Improve test comments and reorganize a bit

* Specify logical pixels

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

* Add framework-side support for system text scale factor.

* Rolling engine to e3404b81a53ba3180c7623a6f2190ebb28518f30
  Additional changes rolled in with engine change:
    libtxt: implementation of GetRectsForRange that processes a line at a time - e3404b8
    Provide an entropy source to the Dart engine (#4161) - e1aa867
    libtxt: search for fallback fonts that can match emoji and CJK characters - 8061df1
    Roll skia to e4679fa06a. (#4157) - 267e7a8
    Update buildroot to 53fea9aebbcc39c6522731471a1a45960ee0685e (#4160) - 02ea7ae
    Revert engine Dart roll. (#4158) - 14aab33
    Add support for system text scale factor. (#4124) - b2a7f4b
    Include _http into sky_engine libraries for analyzer (#4154) - b930f10
    libtxt: Remove postprocess_line and improve tracking of X offsets - 86f95f0
    libtxt: remove redundant line_widths (#4152) - 14bf515
    Roll dart to ade37f931e90b0fdb8fe16d6bf6f089545da55b6 (#4151) - 6f1264f

Also:

* Make TextPainter.preferredLineHeight honour root fontSize

* Remove bogus docs.

* More aggressively track dirty state for RenderEditable.

* Some tests.

Aggressively apply the const lint.

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

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.