Fix hot reload failure handling

Bump Dart SDK to `1.18.0-dev.4.4`.

Updated DrawerHeader to new spec.

Added backgroundImage to CircleAvatar.

Tweaks to --hot mode

This makes the about page show the licenses of all the Dart packages that a Flutter app uses.

Issues that this does not yet resolve:
- I'm still working on getting the full list of licenses for the sky_engine package.
- Some of the licenses don't print very readably.
- There's no scrollbar on the license page.

I'll provide fixes for the first two in the coming days, but this should unblock anyone who is wanting to see something here, even if it's not quite complete. :-)

----

The patch makes the following changes: 

- The license registry is now asynchronous, since the data comes from disk.
- I moved the default license collector from the foundation package to the services package since it uses the default asset bundle now.
- The FLX builder now includes the LICENSE files of each Dart package mentioned in the `.packages` file.

Removed old Stack layout and added a simple-to-extend interface for the
new drawer header. Also added a specialized UserAccountsDrawerHeader
consistent with Material Design guidelines.

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

In order to have an efficient way to display clipped avatars,
backgroundImage was added inside of the container's BoxDecoration.
Fixes #4964. This commit also fixes #4567 where the radius property
actually sets the diamater.

* 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.