Emaultor keeps running on a seuccessful launch, so this automatically returns after 3 seconds if the process hasn't quit (we have to wait for some period to get stderr in the case of a failure).

I can't come up with a better name; anything with Simulator or Device in it will be confused with the existing IOSSimulator/Device classes (which represent the running devices).

`flutter emulators nexus` will now return only those that'd match (eg. same things that `flutter emulators --start nexus` would match).

And it's not obvious to the use what the ID is.

Added an exists check to ensure we don't try to run if emulator is missing, but that requires the file extension for Windows.

Windows sets HOMEDRIVE/HOMEPATH.

It's too fast and leaves weird output on the screen

Very basic support for "flutter emulators" which just lists the available Android AVDs.

Relates to:

https://github.com/flutter/flutter/issues/14822
https://github.com/Dart-Code/Dart-Code/issues/490
https://github.com/flutter/flutter/issues/13379

* Allow customization of TextSpan in EditableText

* Addressed PR comments

* Added test with custom-styled EditableText subclass

* More code style fixes

This replaces the --prefer-shared-library flag, which falls back to
regular (non-shared-lib) compile if the NDK is not found, with the
--build-shared-library flag, which exits with an error message if the
NDK is not found.

This simplifies the set of allowed code paths through AOT compile,
resulting in better testability and easier-to-follow logic. It also
results in more predictable behaviour for continuous integration and
other scenarios.