Use a space instead of a zero-width space to calculate the preferred height of a line of text (#16972)

The zero-width space character may not be supported by the font requested in
the text style.  If that happens, then libtxt will fall back to another font
to render that character, resulting in text metrics that do not match the
intended font.

Fixes https://github.com/flutter/flutter/issues/16257

See https://github.com/flutter/flutter/issues/12630 and
https://github.com/flutter/flutter/issues/16408

Prevent header from thinking it can wrap and then overflowing.

Fix default footer string which lost its colon (localized values are fine).

Make the "rows per page" drop-down include at least one value even when the table lacks many items. (Previously it would assert if your table was too short.)

Make the footer scrollable.

Fix some todos and improve some debug output.

Tests for much of the above.

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

Revert "Revert "Fixes InputDecorator to work with textScaleFactor, fixes Material Design differences. (#12595)" (#12678)" (#12689)

This reverts commit 72dc7d91.

Re-landing my InputDecorator changes so that I can land the character counter PR.

Revert "Fixes InputDecorator to work with textScaleFactor, fixes Material Design differences. (#12595)" (#12678)

This reverts commit 67cf7918.

Reverting because this causes scuba regressions that I'd like to
address in another PR that is pending, but we'd like to roll Flutter.

Fixes InputDecorator to work with textScaleFactor, fixes Material Design differences.

There were a number of differences with the Material Design spec, including
several different padding values and underline thickness.  This corrects
that so that the decorator is in line with the Material Design spec now.

Also, the decorator properly handles changes to the textScaleFactor, where
before it would not re-layout when needed, painting the cursor and
underline incorrectly.

The decorator also now properly animates helper, error, and hint text when
the textScaleFactor or input decoration properties change.

Helper text is now properly displayed in dense mode, as the spec shows.
Before this change, it was never displayed in dense mode.

Fixes #12485

Also:

* Make TextPainter.preferredLineHeight honour root fontSize

* Remove bogus docs.

* More aggressively track dirty state for RenderEditable.

* Some tests.

Also, fix an issue where hint text wasn't visible when the
InputDecoration was collapsed.

Fixes #8541

This patch prepares us to remove InputValue in favor of TextEditingValue.

Covers lib/ in package:flutter.

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

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.

Fixes https://github.com/flutter/flutter/issues/6154

Fixes https://github.com/flutter/flutter/issues/4478

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

Fixes #5873

This commit renames all debugAssertValid methods to
debugAssertIsValid and assert BoxConstaints validity in
constructors. Fixes #5807.

As of `1.18.0-dev-0`, these cases will get flagged.  In the meantime, the

Assigning to `size` called our intrinsic sizing functions re-entrantly, which
is a destructive operation on TextPainter. Ideally we'd made that a
non-destructive operation, but in the meantime we can fix the tab fading issue
by grabbing the text size before assigning to `size`.

Fixes #4365