This attempts to reland #40186 and #41220, that were reverted in #41945.

The main modifications from the original PRs are that I predefine the shortcuts and actions maps instead of defining them inline in the build function, and I use a new mapEquals to do a deep comparison so that we don't rebuild modified things if the contents of the map haven't changed.

I also eliminated an operator== and hashCode that were defined on the Actions widget, since widgets shouldn't have those. (it's too bad though: I get an 85% speedup if we leave this in! Too bad it prevents rebuilding of the children...)

Fixes #40101

* Update DefaultTabController to allow for zero tabs

* add missing trailing commas

* add more missing trailing commas

* add more and more missing trailing commas

* Add Flag to determine if pixels is set by viewport during disposal

* Add TODO to remove nested TabBarView workaround once unnecessary build/dispose issues are resolved

* Re-enable const

* Update TabController to support dynamic tabs.

* Added test for single Tab showing correct color.

This reverts commit 77ab0b83.

* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

* Crash when a TabBar is put in a BottomAppBar.

* Tabs bugs (e.g. crash on transparent material)

- Tabs would crash when placed on transparent Materials
- Tabs would fail to render the child if an icon was specified

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Ensure that the _childElements map is properly traversed as a sparse list and not inflated with garbage collected children.

* Add tests to ensure Lists/Tabs with KeepAlive children can make large jumps, don't lose children (including after rebuild).

Tabs are now padded by 16.0 on the left and right  (used to be 12.0).

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.

This logic is described in the test as looking for a scroll ending
very close to a new page, but in fact its behavior is more like
"very close to a page to the right": if we're not very, very close
to any page, it will pick the page to the left, not an old page.

There's no reason this should be left-right asymmetrical.
Instead, pick the nearest page.

In practice, the case where this makes a difference never arises when
the scroll runs undisturbed to completion; but when the user taps on
the page to hold or drag, the scroll will be interrupted before it
gets within tolerance of a particular page, and this case does arise.

This fixes a glitch that is hard to trigger without time dilation,
but is quite conspicuous with it:

 * Open a tab view with at least 4 tabs, e.g. the Buttons screen
   of the gallery (with "Animate Slowly" on.)
 * Starting at tab 0, tap tab 2.
 * When the animation is nearly complete, tap the page a couple
   of times, as if to drag it around to scroll.  Then let the
   page view settle ballistically toward page 2.
 * Before it finishes, tap tab 3.
 * Suddenly page 1 fills the view, replacing page 2, before we
   scroll from there to page 3.

With this fix, the animation in the last step moves smoothly from
where we are when it starts onward to page 3.

fuchsia_tester.dart still assumes Dart 1. Previously, it ran tests directly
from source, flutter_platform.dart automatically runs a kernel compile when
operating in Dart 2 mode, but this assumes a functional Dart SDK is available
in the artifacts directly, and fuchsia_tester.dart mocks out the artifacts
directory with an empty temp dir.

Remaining work is:
1. Get the frontend server building as a dependency on Fuchsia.
2. Patch fuchsia_tester.dart to use a valid Dart SDK and frontend server.

This also reverts migration to Dart 2 typedef syntax.

This reverts commit 6c56bb24. (#18362)
This reverts commit 3daebd05. (#18316)

Now that Dart 1 is turned off, reapplying my change to turn on the prefer_generic_function_type_aliases analysis option, and fix all the typedefs to Dart 2 preferred syntax.

Also eliminated the unused analysis_options_repo.yaml file and turned on public_member_api_docs in analysys_options.yaml.

No logic changes, just changing the typedef syntax for all typedefs, and updating analysis options.

Revert "Update typedef syntax to use Function notation and turn on lint for old notation. (#18035)" (#18041)

This reverts commit 32586020.

Fixes #18028

Just changes typedef declarations, no logic changes.