This change adds a top-level getBuildDirectory func and funcs for
android, aot, asset, ios build products.
Developers may now add a "build-dir" mapping to their
~/.flutter_settings (JSON format) config file. Output directory is
relative to the main flutter application directory.
This change also changes the default build directory for iOS builds to a
subdirectory of the configured build directory, 'build/ios' by default.
- [x] Update engine to bring in new snapshotter.
- [x] Use the new snapshotter to quickly determine the minimal set of files necessary to run.
- [x] On first DevFS sync, only sync files necessary to run the application.
- [x] Fix a DevFS unit test failure.
- [x] Include DevFS tests in all.dart.
* General improvoments to the loader app:
* Show a message after 8 seconds if no connection comes in.
* Show a progress bar as files are being uploaded.
* Hide the spinner just before launching the application.
* General improvements to the "flutter run" UI:
* Add "?" key as a silent alias for "h".
* Make the help text bold so it doesn't get mixed with the logs.
* Make "R" do a cold restart when hot reload is enabled.
* Supporting features and bug fixes:
* Add support for string service extensions.
* Other bug fixes:
* Expose debugDumpRenderTree() outside debug mode.
* Logger.supportsColor was missing a getter.
* Mention in the usage docs that --hot requires --resident.
* Trivial style fixes.
