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.

* Force upgrade update packages with improved update process.

* Update packages

* Revert "Update packages"

This reverts commit a37de26d3f4f7f5a15746ff791002625faa3d3c0.

* 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

* Generate Android framework config

* Fix templates

* Moar templates fixing

* Minor tweaking

Fixes #11930

* optimize ThemeData: make it monomorphic, memoize result

* address comments

* RLU cache; fix text theme merging

* use FIFO cache for ThemeData; use HashMap to store inherited widgets

* address comments

Eliminates Invocation.typeArguments override recently introduced and removed in upstream Dart SDK commit:
https://dart-review.googlesource.com/c/sdk/+/10702

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)" (#12406)

This reverts commit 90028813.

This change caused a few bots to fail with 'JSON-RPC error 110: Extension error', which is odd because _getUnusedChangesInLastReload is not an extension.

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)

* Make TabBarView/PageView accessible

* review fedback

* update the intellij doctor check to support the m18 plugin version

* update to support multiple package names

Documentation changes, and, in particular, the ability to lerp from
and to any decoration by lerping via null.

Add hidden options --extra-front-end-options and --extra-gen-snapshot-options to flutter tool (#12219)

This CL introduces 2 hidden options to 'flutter build aot' and 'flutter run' for passing arbitrary arguments to front-end server and to gen_snapshot tool when building and running flutter app in --profile or --release modes.

The ability to pass arbitrary options simplifies various experiments, as it removes the need to change defaults and rebuild flutter engine for every tested configuration.

* 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

Previously, we used `Alignment`, which was difficult to understand. Now,
we just use an `Offset` scaled to the child's size, which is much easier
to understand.

These now act the way they used to act if both operands are
FractionalOffsets.  Once you mix in some other AlignmentGeometry
objects, everything gets converted to the AlignmentGeometry coordinate
system.

FlutterLogoDecoration should defer to Decoration for what to do when
it doesn't know how to lerp to/from another kind of decoration.

I'm about to add the BoxBorder and BorderDirectional classes to
this new file, but figured it would make review easier if the move
of the existing class happened first.

This reverts commit f6fe8dc7.

This breaks coverage test
https://travis-ci.org/flutter/flutter/jobs/282472585

* Roll engine

* Pick up updated engine with analyzer fix

* Add new typeArguments override

* Update engine dep

* Up dartdoc version to fix name resolution issues

* 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

Reorder the code, remove some redundant documentation, improve the
interpolation logic.

This class lays the groundwork for RTL borders.

* ++

* Allow unmerging of SemanticsNodes

* test passing

* ++

* remove prints

* doc comments

* rectify comment

* review comments

Alignment will eventually replace FractionalOffset.

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.