Fundamentally the core problem was that we were not saying how wide a
date picker should be. It should be 330 pixels, if I'm measuring the
spec's mocks correctly.

Fixes https://github.com/flutter/flutter/issues/4910

This patch adapts the AppBar to feel more like iOS by centering the title.

Fixes #4962

We should center the title of flexible space bars on iOS.

Related to #4962

* This reverts commit 5e7e7b6e.

* default to lib/main.dart

BUG=https://github.com/flutter/flutter/issues/4904

This reverts commit 32ad81c8.

We'll use this field to adapt material widgets to iOS.

If the scaffold is new and has a floating action button, we skip the entrance
animation for the fab.

Fixes #4445

This patch includes a number of improvements:

 * Material page routes now put a repaint boundary inside their transition so
   they don't repaint during the transition.
 * Heroes that are on a quest now get a repaint boundary so we repaint them
   individually.
 * I've hoisted the transparent material for the product items up in the widget
   tree, which doesn't affect performance but makes the ink splashes reach the
   edge of the product cards.
 * I've changed the repaint rainbow visualization to make it easier to see
   what's going on.

Previously, we considered them one operation in the compositor, which didn't
trigger caching. Now we have a way to explicitly hint that the compositor
should cache a layer.

Fixes https://github.com/flutter/flutter/issues/4996

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.