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.

Fixes https://github.com/flutter/flutter/issues/11041

And add a test to verify we don't do this again.

https://github.com/flutter/flutter/issues/10203

* Only one call to createSnapshot exists, and it's in the same library.

* Eliminate conditional logic around the presence of depfilePath, the
  only existing call always passes a non-null depfilePath.

* Output verbose cocoapods output when pod install has an error or if flutter build/run is in -v

* remove repeated stdout

This adds a way to run only a subset of the tests.
(The new flags do the same thing as 'pub run test'.)

* increase the timeout for service discovery

* remove the timeout for discovering service ports

Remove terminating semicolons; they are causing an "code inspection warning" in Android Studio:

```
This inspection reports redundant semicolon (';') token which is not required in Kotlin and may be removed.
```

* Guess sign with the first certificate when multiple are available in machine mode

* review

In the case where the CocoaPods 'pod install' step fails, emit its
stdout to provide any potentially-useful error messages to the user.

This reverts commit b800654a.

This time, do so in Travis' sudo-enabled environment,
which is said to be less prone to networking issues.
It also means we'll run in an env with 7.5GB of RAM
instead of 4GB or RAM (at the cost of about 45 seconds
of extra boot-up time).

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

This reverts commit aa904a58.

Fixes #10940

* Switch existings tests to use equalsIgnoringHashCodes and add more tests.

* Use defaultTargetPlatform instead of Platform.operatingSystem for ImageProvider.
Add convenience helper to make it easier to display a short string for
an enums value.

* Add comment linking to issue.