DevFS asset support and deletion support

Update Dart (1.19.0-dev.0.0) and analyzer (0.27.4-alpha.19).

Also, add a few more debugging flags.

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`.