Fixes #2569

Enable always_declare_return_types lint

And fix a zillion omissions this uncovered.

[engine] bump version to f2d8d75897a5aadcd9be0067a78fce1cea677fd6

Make flutter analyze useful for package conflicts

Also, resolve our package conflict, since reflectable has been fixed.

Not as

It has performance implications in release mode.

We'll enable the lint once https://github.com/dart-lang/linter/issues/195 is fixed.

Improve InputValue#toString

Test harness should check for running Timers and AnimationControllers

After running a widget test, we now clear out the widget tree and check that we
didn't leak any timers or animations.

Also, fix several bugs that this addtional check revealed.

Fixes #2481

Adds initial version of Markdown renderer

Add `flutter_tools build` hooks to also generate depfile.

fix a regression in flutter logs

Fixes #1942.

Use lowerBound() from package:collection/collection.dart

Added SizeTransition

fix a npe in lastLogcatTimestamp

Remove //examples/widgets

Most of the remaining widget examples are actually manual tests. This patch
moves them into //dev/manual_tests. A couple are examples of using services,
which I've moved to //examples/layers/services. The remainder are out-dated and
are removed by this patch.

Support undo in the leave-behind demo

Support reparenting state from MultiChildRenderObjectElement

Add ServiceProtocolDiscovery with tests

Fixes #2085

Drop the argument to SceneBuilder's constructor

CheckedPopupMenuItem now fades its icon when changed.

BUG=https://github.com/flutter/flutter/issues/2274

Update engine.version

RenderCustomMultiChildLayoutBox shouldn't be sizedByParent