* Keep Icon's subtree structure the same with and without a semanticLabel.

This avoids rebuilding the subtree when a semanticLabel is set/unset.
Also updated some doc comments.

(This is a followup on post-merge comments for #12475)

* Allow specifying and a11y label for Icon widget

Also:

 * Provide a better message when you lerp from infinity to finity
   constraints.

 * Make the striped marker support RTL.

 * By popular demand, dump a warning to the console the first time
   a particular Flex overflows. (Resets on hot reload.)

Change PageController#nextPage & #previousPage methods to return Future when the animation completes. (#12299)

* Changed PageController #nextPage & #previousPage to return a Future after completing.

* Removed false statement about returning a Future in PageController#jumpToPage

* Added tests to make sure nextPage & previousPage return Futures.

* Tested that the Futures returned by nextPage & previousPage actually resolve.

* Renaming *pageCalled to *pageCompleted, since that's what it's actually for.

* formatYear

* localize date picker

* tests

* clean-ups

* address comments

* Revert "When parts of the program are changed in a hot reload, but not executed during the reassemble, warn that a restart may be needed. (#12304)"

This reverts commit 90028813.

* fix _debugCheckOwnerBuildTargetExists; sync localizations and tests

* address comments

Fixes #11930

* Make TabBarView/PageView accessible

* review fedback

* Drop invisible SemanticsNodes from tree

A node is invisible if it is outside of the bounds of the screen and if it is not merged into its (partially) visible parent.

Also in this PR: only set `wasAffectedByClip` to true if the nodes has actually been clipped.

* Fix other failing tests

* renaming

* review feedback

* more doc

* 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

* ++

* Allow unmerging of SemanticsNodes

* test passing

* ++

* remove prints

* doc comments

* rectify comment

* review comments

Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.

In 85c425ac, a test was added to ensure that widget.onChanged was
fired when the contents of an EditableText changed via system paste
events (e.g. triggered from a TextSelectionOverlay). Due to a long stack
of rebases and (less-than-perfect) manual merge conflict merge
resolution, it was inadvertently added twice.

This patch fixes a collection of issues with widgets involved in text
editing:

  * Fire widget.onChanged on EditableText value change:
    The value of an EditableText is composed of the text value as well
    as other editing-related data such as selection-related information.

    Previously, widget.onChanged() was only called for updates via
    updateEditingValue(). For pastes via a TextSelectionOverlay, updates
    are signalled via _handleSelectionOverlayChanged(), which only ever
    triggered widget.onSelectionChanged(), but not widget.onChanged().

    Both updateEditingValue() and _handleSelectionOverlayChanged()
    perform the value update via _formatAndSetValue(), which is where
    this patch moves the widget.onChanged() call.

  * Correctly update TextFormField value on edits via controller:
    The textual value of a TextFormField exists in two locations:
      1. FormField.value, as with all FormFields and subclasses.
      2. TextEditingController.value associated with the TextField
         underlying the TextFormField.

    Previously, edits to the TextEditingController associated with a
    TextFormField resulted in updates to the rendered TextField widget,
    but did not update TextFormField.value. FormField.value is updated
    via FormField's onChanged function, which is called from the
    EditableText underlying the TextField underlying the TextFormField.
    EditableText only fires onChanged when it receives changes from the
    engine. It does not fire onChanged for changes made to the
    underlying TextController, since the owner of the TextController is
    the one making these changes and thus, already aware of them.
    FormField, however, *does* need to listen to these changes to update
    its value.

  * Adds an initialValue parameter to the TextFormField constructor:
    FormField's constructor already takes an initialValue parameter,
    which specifies the initial value in the field, which is also the
    value to which reset() returns the field.

    Previously, TextFormField took its initial value from the controller
    value (if a controller was passed in) or the empty string (if not).
    This had the undesirable effect that calling reset() always resets
    the value to the current value of the controller... i.e., does
    nothing.

    We now take an initial value explicitly.

* SafeArea

* AnimatedSafeArea

* AppBar test

* Apply feedback

Adds a test that verifies that EditableText sends a
TextInput.setEditingState message to the engine when the associated
TextEditingController is replaced.

Add multiline keyboard support to editable text widget.  Fixes #8028.

* Send scroll progress with ScrollCompletedSemanticsEvent

This requires engine change https://github.com/flutter/engine/pull/4144

* fix analyze warning

* review comment

* Roll engine to 45b11f742d38ebf564a5a832b1af00661d1a31fa

* fix test

This widget makes it easier to animated Positioned widgets with
awareness of the ambient Directionality.

Fixes #11998

Also, fix asserts in TableBorder#paint to match documentation.

Fixes #12009

I inflated instead of deflating. Oops.

* Record original pointer event timestamp

* review

* review

* review

* Matched motions with iOS. Didn’t add overscroll spring clamps and fix tests yet.

* clamp max overscroll transfer

* Add test

* review notes, moved things around

* remove function passing indirection

* review

* Replace stopwatch with timestamp from #11988

* move static

* Review

This will enable both to be RTL'ed.

Also, factor out common border painting code into paintBorder.
Also, make Border paint uniform non-rounded borders using drawRect.
Also, add some documentation about an issue that wasted an hour of my life.
Also, factor out all the border painting code into TableBorder.paint.

This reverts commit da0602fe.

Adds a default title to the Title widget, fixes #6802.

* Show accessibility frame while scrolling

* ++

* fix test

Diagnostics object is.