* System chrome overlay style fixes

1) Add a cache to avoid superfluous calls to the embedder
2) Coalesce calls to the embedder that are made in the same event loop
3) Move call site to material's AppBar from Title widget

#4461

* bump out dep on the test package

* add other pubspecs

Fixes #4553

I broke them when refactoring images. Oops.

Fixes #4169 and also now overscroll indicator is dismissed the moment
user scrolls in the opposite direction (#4603) but the bounce overscroll
that happens behind the scene and is clamped in the indicator is still
problematic and needs to be fixed. However these fixes are orthogonal to that.

Also closes #4127 as I verified the timeout feature (reduced the duration
to 500ms to be closer to Android behaviour)

The problem was that the Scaffold was getting a new key each time we
navigated to the main page. The key influences where page state is
stored, including the scroll offset. So for a single frame, the Scaffold
incorrectly thinks the scroll offset is 0 and the app bar should be
expanded.

BUG=https://github.com/flutter/flutter/issues/4400

Dead code, switch cleanup and `crypto` bump.

Overview
========

This patch refactors images to achieve the following goals:

* it allows references to unresolved assets to be passed
  around (previously, almost every layer of the system had to know about
  whether an image came from an asset bundle or the network or
  elsewhere, and had to manually interact with the image cache).

* it allows decorations to use the same API for declaring images as the
  widget tree.

It requires some minor changes to call sites that use images, as
discussed below.

Widgets
-------

Change this:

```dart
      child: new AssetImage(
        name: 'my_asset.png',
        ...
      )
```

...to this:

```dart
      child: new Image(
        image: new AssetImage('my_asset.png'),
        ...
      )
```

Decorations
-----------

Change this:

```dart
      child: new DecoratedBox(
        decoration: new BoxDecoration(
          backgroundImage: new BackgroundImage(
            image: DefaultAssetBundle.of(context).loadImage('my_asset.png'),
            ...
          ),
          ...
        ),
        child: ...
      )
```

...to this:

```dart
      child: new DecoratedBox(
        decoration: new BoxDecoration(
          backgroundImage: new BackgroundImage(
            image: new AssetImage('my_asset.png'),
            ...
          ),
          ...
        ),
        child: ...
      )
```

DETAILED CHANGE LOG
===================

The following APIs have been replaced in this patch:

* The `AssetImage` and `NetworkImage` widgets have been split in two,
  with identically-named `ImageProvider` subclasses providing the
  image-loading logic, and a single `Image` widget providing all the
  widget tree logic.

* `ImageResource` is now `ImageStream`. Rather than configuring it with
  a `Future<ImageInfo>`, you complete it with an `ImageStreamCompleter`.

* `ImageCache.load` and `ImageCache.loadProvider` are replaced by
  `ImageCache.putIfAbsent`.

The following APIs have changed in this patch:

* `ImageCache` works in terms of arbitrary keys and caches
  `ImageStreamCompleter` objects using those keys. With the new model,
  you should never need to interact with the cache directly.

* `Decoration` can now be `const`. The state has moved to the
  `BoxPainter` class. Instead of a list of listeners, there's now just a
  single callback and a `dispose()` method on the painter. The callback
  is passed in to the `createBoxPainter()` method. When invoked, you
  should repaint the painter.

The following new APIs are introduced:

* `AssetBundle.loadStructuredData`.

* `SynchronousFuture`, a variant of `Future` that calls the `then`
  callback synchronously. This enables the asynchronous and
  synchronous (in-the-cache) code paths to look identical yet for the
  latter to avoid returning to the event loop mid-paint.

* `ExactAssetImage`, a variant of `AssetImage` that doesn't do anything clever.

* `ImageConfiguration`, a class that describes parameters that configure
  the `AssetImage` resolver.

The following APIs are entirely removed by this patch:

* `AssetBundle.loadImage` is gone. Use an `AssetImage` instead.

* `AssetVendor` is gone. `AssetImage` handles everything `AssetVendor`
  used to handle.

* `RawImageResource` and `AsyncImage` are gone.

The following code-level changes are performed:

* `Image`, which replaces `AsyncImage`, `NetworkImage`, `AssetImage`,
  and `RawResourceImage`, lives in `image.dart`.

* `DecoratedBox` and `Container` live in their own file now,
  `container.dart` (they reference `image.dart`).

DIRECTIONS FOR FUTURE RESEARCH
==============================

* The `ImageConfiguration` fields are mostly aspirational. Right now
  only `devicePixelRatio` and `bundle` are implemented. `locale` isn't
  even plumbed through, it will require work on the localisation logic.

* We should go through and make `BoxDecoration`, `AssetImage`, and
  `NetworkImage` objects `const` where possible.

* This patch makes supporting animated GIFs much easier.

* This patch makes it possible to create an abstract concept of an
  "Icon" that could be either an image or a font-based glyph (using
  `IconData` or similar). (see
  https://github.com/flutter/flutter/issues/4494)

RELATED ISSUES
==============

Fixes https://github.com/flutter/flutter/issues/4500
Fixes https://github.com/flutter/flutter/issues/4495
Obsoletes https://github.com/flutter/flutter/issues/4496

# Conflicts:
#	packages/flutter/pubspec.yaml

Prep to get us ready to pull in a new dev SDK and bump our analyzer DEP.

* updates `crypto` (required by fresh analyzer)
* fixes newly flagged dead code warnings
* fixes switches that fall through and don't return

I got confused about the sdk_ext directory.

Prep to get us ready to pull in a new dev SDK and bump our analyzer DEP.

* updates `crypto` (required by fresh analyzer)
* fixes newly flagged dead code warnings
* fixes switches that fall through and don't return

I moved dart_ui.dart to an new path and forgot to update this reference.

FAB segue animation is done in scaffold and the consts are there.

* send application files to the observatory's devfs

* update comment

Fix the switch to match the Material Design spec.

Our track and head were slightly off in size.
These now exactly match the sizes in
https://material.google.com/components/selection-controls.html#selection-controls-switch
We took the liberty of assuming the head overlaps the edge
of the track by 2 pixels (instead of the head and the track ending at the same point)
which the provided designs were conflicted about.

Huge thanks to @DaveShuckerow for noticing this issue!

@abarth

* Fix `flutter upgrade`

* Fix builds on iOS

Add missing returns.