Previously we triggered an assert.

Fixes #4850

We were stacking up two dismiss timers. Now we don't start a new timer
if we've already got one. Also, fix an animation leak caught by the
test.

Fixes #4640

Rather than requiring the developer to specify a full Decoration, we now
just take an ImageProvider for the thumb image. Also, fix
ImageFit.scaleDown to actually work.

Fixes #4571
Fixes #4673

Also, fix an animation leak found by the test.

Fixes #4619

We now have an EdgeInsets.along function that projects the padding onto the
given Axis rather than repeating this function in several places.

We want to use the FlutterViewController in this new project template.

Fixes #4840

* Make the size setter on RenderBox protected to avoid setting from
   outside the class.
 * Make normalize on BoxConstraints always return an object that
   isNormalized.

Fixes #4761

Bump Dart SDK to `1.18.0-dev.4.0`.

Last dev push for `1.18`.

We now use a different approach for ios projects where the developer controls
the Xcode project file. This patch removes the old ".generated" approach in
favor of the new approach.

This patch changes the default `flutter create` command to generate a
HelloServices-style app on iOS. As a consequence, the Atom integration now
fully works with HelloServices.

This came up in security review.

@abarth

* send ext.flutter.debugDumpApp; ext.flutter.reassemble

* expose debugDumpRenderTree

We now stop updating semantics when there are no remaining clients.

Host apps will return null if there is no matching handler for a message name

Also, make SemanticsOwner into a real class and use it instead of a static in
several places.

Now that protected can be accessed from inside the same library, we can
use protected in a number of new places and we can remove some
trampolines we were using to work around its previous semantics.

This patch updates the framework to use the new SemanticAction in
semantics.mojom.

It can cause trouble if the user has a newer Android SDK installed normally.
Instead, just use the normal Android SDK.

navigating away.

Details are in the bug, but when we navigate away, the overscroll
indicator around the Scrollable would change the widget hierarchy. This
would trigger the Scrollable to rebuild, which would cause its
scrollOffset to be clamped to 0 for a frame.

BUG=https://github.com/flutter/flutter/issues/4597

Update to latest analyzer (w/ cleanup).

Bye-bye `@protected `trampolines! :)

Update to latest Dart SDK (1.18.0-dev.3.0).

Our previous approach to detecting when we needed to remove semantics nodes
didn't account for reparenting.

Cleanup empty statements.

The new `empty_statements` lint (in the next DEV roll), flags these empty statements.  Harmless, except the one in `basic.dart` that fixes a real-life bug. :)