BUG=https://github.com/flutter/flutter/issues/5351

* Udpate to Dart `1.20.0-dev.1.0`.

Udpate to Dart `1.20.0-dev.1.0` and corresponding `analyzer` package.

* Added `--no-packages-dir`.

* Revert extension to master.

* Added ignores.

* Review nits.

* More nits.

This issue got fixed by a recent check-in, but I had a test specifically
for this incarnation so we should probably keep it just in case.

Closes https://github.com/flutter/flutter/issues/5630

Update engine revision to pull in fixes for crashes on arm64 due to missing CRC32 instruction. (#5774)

This updates the flutter analyze command to use the .analysis_options_flutter_analyze in the repo root when either the --flutter-repo flag is supplied or the sources being analyzed reside in the flutter repo.

Previously, if a StatefulWidget was marked dirty, then removed from the
build, then reinserted using the exact same widget under a widget under
a LayoutBuilder, it wouldn't rebuild.

This fixes that.

It also introduces an assert that's supposed to catch SizeObserver-like
behaviour. Rather than make this patch even bigger, I papered over two
pre-existing bugs which this assert uncovered (and fixed the other
problems it found):

   https://github.com/flutter/flutter/issues/5751
   https://github.com/flutter/flutter/issues/5749

We should fix those before 1.0 though.

In the interest of using the name in the case of Material steppers,
this commit renames Step curve to Threshold.

This eliminates all the issues around the horizontal page transition. I also improved the back gesture animation, and added fling support.

BUG=https://github.com/flutter/flutter/issues/5678
BUG=https://github.com/flutter/flutter/issues/5622

Related: b5f763b4

Fixed a bug where the size of the AnimatedCrossFade would always
start from the size of the first child, irrespective of the
initial crossFadeState argument.

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