Add --hot mode for flutter run with Android devices and iOS simulators

* instructions for building the gallery

* tweaks from review

* tweaks from review

so that folks have some prayer of figuring out from our docs
how to turn it off. :)

@Hixie

* making flutter run work with gradle

* locate android studio

* add test for settings

* review comments

The appbar now scrolls off screen when you scroll the page far enough.

BUG=https://github.com/flutter/flutter/issues/4410

This reverts commit 6070cd94, reversing
changes made to d0f43ed9.

It broke the waterfall.

Update to latest Dart SDK (1.18.0-dev.4.2).

* update flutter run --devfs

* use async; add todo

Fixes https://github.com/flutter/flutter/issues/4407

Previously it wasn't included in the Runner target.

* Driver commands for controlling the Input widget

This commit introduces two new driver commands for controlling the
material Input widget.

* setInputText(SerializableFinder finder, String text)
* submitInputText(SerializableFinder finder)

Since it is not possible to directly modify the Input widget text,
these driver commands invokes the handler functions of the Input
widget: onChanged and onSubmitted, respectively. The `submitInputText`
command returns the submitted String as a result.

* Make input command handler methods private

Addressing comments from @yjbanov.

Simplify iOS template project

Fall back to the default xcodeproj if a workspace cannot be found

Remove dependency on FlutterAppDelegate so devs can more easily add SDKs.

When using flutter run, build the workspace instead of the project.

* configs for firebase hosting

* use the implicit FIREBASE_TOKEN

* don't print every command (lowers the chance of printing secret env vars, and for realz only deploy when merging into master