Apply a 30 second timeout to Android/iOS device polling.

If there's a device poll already in progress, skip polling for new
devices; wait for the first request to return/timeout.

Was broken by https://github.com/flutter/flutter/commit/dba29aa0fce684b201c6b03efc73ba607c7b507b, which apprently has been pushed directly to master without running through Travis?

Fixes #9413.

For more info on conditional imports, see
https://github.com/munificent/dep-interface-libraries/blob/master/Proposal.md.

Github makes own copies of images referenced in README, and that fails for large image sizes. The gif has 6MB and so is over the threshold.

Instead of using an off-GitHub image, use the same image uploaded to user-images.githubusercontent.com. This makes GitHub show the image even when it's very large.

This reverses https://github.com/flutter/flutter/pull/11170 and finalizes https://github.com/flutter/flutter/pull/11158.

Github makes copies of images referenced in README, and it fails for large image sizes. The gif has 6MB and so is over the threshold.

This removes the image until I can figure out how to reinsert it so that it shows.

Reuses flutter.io homepage content to give README.md an upgrade, since this file will be the shop window of Flutter to a large part of newcomers. 

The downside is that we now have duplicate content that should be updated in unison with the main site but which cannot be easily copy-pasted, due to constraints of github's Markdown renderer.

* requires bumping `process` to a version that has
  relaxed version restrictions on `intl`

https://github.com/flutter/flutter/issues/10650

* Don't pollute Travis log with Google Cloud SDK installation

* review comments

* a11y fix for list_demo.dart

* fix semantics of persistant bottom sheet

* add Android instrumentation test

* add devicelab test

* add to manifest.yaml

* rename _smoke_test.dart to _smoketest.dart to prevent flutter test from picking it up

* volatile fields; style fixes

* use ConditionVariable; fix sh script

Fixes #7894

The output of the analyzer gives the full path to the Dart file, so
we had to adjust the string matching to account for that.

https://github.com/flutter/flutter/issues/7894

* Include the process' `stdout` and `stderr` when it returns a
  non-zero exit code in `runCheckedAsync()`
* Defensively catch errors in `AndroidDevice.isAppInstalled()`
  and return false

* report run target and if it is an emulator

* don't print debug

* rename parameter, remove unused variable

* fix test

* fix comment

* tweak from review, and fix analyzer error

* send custom parameters for the event, not the session

* fix mock

* use the +1 for usage

* add a --machine mode to flutter config

* review feedback

* minor tweaks to the output for flutter_tools

* update test expectations

https://github.com/flutter/flutter/issues/10940

* Revert "test missed a mock (#11098)"

This reverts commit 1a01de9a.

* Revert "Roll engine to 9117705b3653565a240314481c63eb93090f7232 (#11114)"

This reverts commit 12b39528.

* Exclude modal barrier from semantics on Android

* Add tests

* Make the drag resistance non-linear

* Let the easing of overscroll have a spring effect too

* Add tests and prevent possible drift by having a slightly smaller resistance when easing the overscroll

* lint

Snapshot dependency files now backslash-escape dependency paths
containing spaces and backslashes.

See: https://codereview.chromium.org/2966903003/

Previously, the snapshot file was recomputed on every build. We now
record checksums for all snapshot inputs (which are catalogued in the
snapshot dependencies file output alongside the snapshot) and only
rebuild if the checksum for any input file (or the previous output file) has
changed.

See https://github.com/flutter/flutter/issues/11051

(Needs https://github.com/dart-lang/sdk/issues/28519)

* AutomaticKeepAlive

A Widget that listens for notifications from widgets that don't want to die.

* Automatically wrap SliverList and SliverGrid children in AutomaticKeepAlive widgets

* Fixes for review comments

Previously, the snapshot file was recomputed on every build. We now
record checksums for all snapshot inputs (which are catalogued in the
snapshot dependencies file output alongside the snapshot) and only
rebuild if the checksum for any input file has changed.