Closes #5702

Update transitions_perf_test.dart

Update simple_smoke_test.dart

Fixes #5393.

* adjust channel_test to be less specific
fixes https://github.com/flutter/flutter/issues/5716

* show device list in flutter doctor output
fixes https://github.com/flutter/flutter/issues/5625

Fixes https://github.com/flutter/flutter/issues/5234

The iOS transition is the more buggy part of this feature, and it wasn't
disabled. Let's just leave them enabled while I work on improvements.

* improve device matching error message
fixes https://github.com/flutter/flutter/issues/5675

One of the drawer items was dsiabled but still clickable which
led to inconsistent icon coloring. THis commit makes this item
actionable. Fixes #5683.

Subheading title was too close to the actual example. This commit
increases the size of its box. Fixes #5682.

The floating action button inside of the Pesto demo now opens up
a snackbar. Fixes #5685.

BUG=https://github.com/flutter/flutter/issues/5599

Implement flutter format, fixes https://github.com/flutter/flutter/issues/5338

Added a widget that cross fades two children while animating the
size of the parent based on the children's interpolated sizes.

Fixes https://github.com/flutter/flutter/issues/5592

See https://github.com/flutter/flutter/issues/5570

Also added a regression test for the back gesture on iOS and android.

* fix getDeviceById to match exact name
fixes https://github.com/flutter/flutter/issues/5508

* peg dartdoc to a specific version

* add some documentation

* apply the fix to dev/bots/docs instead

Previously, it was unclear that the middle slider was disabled.

Fixes #5517

Fixes https://github.com/flutter/flutter/issues/5283

Other changes in this patch:

Rename OffStage to Offstage.
Fixes https://github.com/flutter/flutter/issues/5378

Add a lot of docs.

Some minor punctuation and whitespace fixes.

1: If a route is already dismissed when it's popped, there's no point
trying to animate heroes, because it's going to be gone before the
heroes code can look at it.

2: If a hero animation finishes just as a new one is starting, we
previously blew away the state for the starting one. Now we correctly
segregate the "starting up quest" variables from the "actively ongoing
quest" variables.

Added ExpansionPanel and ExpansionPanelList. The implementation
is based on the Material design spec, without enforcing every
facet of it, leaving the developer using this the liberty to
enforced it lower down the widget tree. This decision was made
based on the argument that implementing more of the spec would
reduce flexibility to a point where some design would not be
implementable with this widget anymore.

Somehow we missed this before.

Fixes https://github.com/flutter/flutter/issues/5615

Fixes https://github.com/flutter/flutter/issues/5588