In Dart 2 mode we throw different exception: compiler generates
throw of a compilation error instead of runtime throwing
noSuchMethod.

This cleans up lerp, lerpFrom, lerpTo, and scale methods, and their
documentation.

Fixes https://github.com/flutter/flutter/issues/12377.

* Theme provides all TextStyle properties

* match field declaration order in the test

* Theme.of returns text style with inherit == false

* change TextStyle.inherit logic; docs

* add TextStyle.debugLabel

* address comments

* add debug labels to Typography text styles

* 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

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

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

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.