(and minor doc changes)

This will in the future allow Scrollable to not track the Simulation itself.

More idiomatic use of constraints in performResize.

Trivial fixes to comments.

Make ProxyBox not use BoxParentData since it ignores the field.

Make applyPaintTransform more helpful if you use a different ParentData
subclass than RenderBox expects.

Make debugAssertIsValid actually fulfill its contract in RenderObject as
documented.

Add a childBefore for symmetry (we already had childAfter).

Fix the way we dump the child list when there's no children in a
multichild render object.

More asserts in the rendering test library.

We now center the body correctly and handle Fuchsia key events. Also, add
support for modifiers.

See #6631

These define a TextTheme and IconTheme that contrast with the accent
colour brightness. Also adjust default accentColorBrightness to match
Material spec examples (dark text/icons on teal in Dark theme).

Update material components to use accentTextTheme, accentIconTheme:
* DatePicker selection
* Floating action button icon
* TimePicker selection
* Slider label text

Fixes https://github.com/flutter/flutter/issues/6603

People get confused by the tight constraints.

On Android and Fuchsia, default to Roboto. On iOS, use San Francisco.

FormField is now a widget that can contain any type of field. Input no
longer has special code to handle form fields. Instead, there is a
helper widget InputFormField for using an Input inside a FormField.

Fixes https://github.com/flutter/flutter/issues/6097 and based on
feedback from the same.

We now create the Completer inside TransitionRoute, as with the popped
completer. Also, test that this future resolves at the appropriate time.

Fixes #6591

If a recognizer is interrupted by an exception from a callback, it could be
left in an inconsistent state and be unable to process future events

The theme fallback mechanism was busted by the shadowThemeOnly change.

Fixes #6562

Fixes https://github.com/flutter/flutter/issues/6451

If a Container with no child is in an infinite space, make it shrink
instead of expanding. Still expand otherwise.

This patch removes our dependency on asset_bundle.mojom.

This patch removes uses of keyboard.mojom in favor of the
TextInputPlugin.

If maxLines is 1, it's a single line Input that scrolls horizontally.
Otherwise, overflowed text wraps and scrolls vertically, taking up at
most `maxLines`.

Also fixed scrolling behavior so that the Input scrolls ensuring the
cursor is always visible.

Fixes https://github.com/flutter/flutter/issues/6271

They support multiline text now.

This class will eventually replace the Keyboard class we currently use. As part
of this migration, we'll switch from using mojom to interact with the IME to
using platform messages.

The platform plugin now supports clipboard operations.

Fixes #616

Developers need to get the size of the BuildContext sufficiently often
that we should provide a convenient getter for the value. Having this
getter is also an opportunity to catch common mistakes and provide
useful error messages that guide developers towards better patterns.

Fixes #2321

Fixes #6245

Previously, we were nulling out the ArenaEntry in MultiDragPointerState
too early, which was prevent MultiDragPointerState from rejecting the
gesture in `dispose` if we hadn't accepted by the time the pointer went
up. Now we properly reject the gesture, which causes the tap gesture to
win during `sweep` in the arena.

Also, add a bunch of docs and annotations based on information I learned
while studying this issue. Finally, rename a private member of tap
recognizer to a name that would have confused me less in my
investigation.

Fixes #1186

When I changed how routes complete their futures, I broke the Dropdown
button because it was still waiting for its own Completer to complete
instead of using the Future returned by push. This patch fixes that
issue.

I've also removed the previous behavior of the DropdownButton forwarding
its text style to its route. The mechansim that we were using doesn't
work properly in all cases. For example, if the DropdownButton is a
child of a LayoutBuilder, then the route will have already built by the
time the DropdownButton gets a chance to forward its text style, causing
an assert in setState.

Finally, I've tweaked PopupMenuButton to work the same way as
DropdownButton in a couple corner cases (e.g., not calling the changed
callback if the button was removed from the tree before the menu
completed its Future).

Fixes #6352