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

ImageFit.cover doesn't respect alignment

We need to apply the alignment to the source rect because the ouput rect fills
the destination rect.

Fixes #1514

Be more verbose when downloading artifacts

The getSize function from MultiChildLayoutDelegate might depend on information
other than the incomming constraints.

Fixes #2077

Remove bogus intrinsic size assert

This patch tweaks the definition of max intrinsic size to give implementations
more wiggle room to compute there intrinsic sizes more efficiently.
Specifically, implementations can now ignore the incomming constraint in the
cross axis if they prefer.

This patch removes an assert that is no longer valid w.r.t. the new definition
of intrinsic sizing.

Fixes #2099

Refactor DeviceLogReader

More dartdoc for routes.dart

Assert binding ready before getting a11y listener

Previously, we would show 4 progress bars, which made it very unclear
what was going on since we'd just said "downloading engine". Now each
separate thing we download has its own announcement.

Eventually we should probably only download Dart, then do the rest of
this inside the flutter tool, as necessary.

This adds an assert to verify that we have already got a binding when we
get a semantics listener. Otherwise we crash. (I mean, we still crash,
but now in checked mode we crash on an assert instead. Progress?)

Added support for Dismissable leave-behind list items

remove the _embedder.yaml file

implement --watch for flutter analyze

Cleanup around viewports

Cleanup build