Previously we would rebuild every route on the second animation frame to
rejigger the offstage bit and the animations. Now we build the page once and
update the offstage bit in place and update the animations using
ProxyAnimations.

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