This patch replaces uses of Flexible with Expanded where we're using
FlexFit.tight. We still need to think of a better name for the
FlexFit.loose variant.

Also, improve the docs for Row, Column, Flex, and RenderFlex to be more
problem-oriented and to give a complete account of the layout algorithn.

Fixes #6960
Fixes #5169

Fixes https://github.com/flutter/flutter/issues/6561

Since we can change the theme's platform, we need to make sure we
propagate that throughout rather than having half the app use the native
platform and half the app use the selected platform.

The current implementation matches the default behaviour of the material
scaffold.

* Changed Pesto logotype's position.

This commit changes Pesto's logotype position so that its curve
aligns with the curve of the image size.

* Added clamping to Pesto logo animation.

Fixes https://github.com/flutter/flutter/issues/5711

Removed Pesto logo's triggered animation in order to remove the
observable 'hop' when scrolling. The whole curve is now entirely
scroll dependent. Fixes #5907.

Due to a synchronization issue, a PR was merged with a green build
that was supposed to be rebuilt and to fail. This commit fixes the
issue by renaming the usage of a constant.

Fixes https://github.com/flutter/flutter/issues/5684

* Added OverscrollIndicatorEdge et al

* RefreshIndicator only clamps its scrollable edge

* added a test

* Updated the test

* fixed lint-os

* fixed a typo

* Scrollable should restore its viewport dimensions when it reappears

* removed an accidental commit

* updated per review feedback

The drawer was confusing without adding much value. It was
especially confusing when wanting to go back to the Gallery on iOS.
Fixes #5713.

BUG=https://github.com/flutter/flutter/issues/5351

The floating action button inside of the Pesto demo now opens up
a snackbar. Fixes #5685.

Fixes https://github.com/flutter/flutter/issues/5462

@mpcomplete

BUG=https://github.com/flutter/flutter/issues/5098

This patch improves the Post and Shrine transitions by making the AppBar
into a Hero and changing the default MaterialPageTransition. Now the
AppBar transitions smoothly between screens and the
MaterialPageTransition doesn't involve a fade effect.

Also, rejigger the bounds of the image header in Pesto to avoid the
"pop" at the end of the animation by laying out the image header at its
final visual size instead of relying on occlusion to size the image
header.

Fixes #5202
Fixes #5204

Previously we were resizing a paragraph of text during the animation. Now we
animate the text and the image separately. Also, add a default hero tag for
FloatingActionButton so that it animates as part of the hero transition as
well.

These now have sorter names to make the callers less verbose.

These let you add Align widget to the inside or outside of a container.
Several customers have asked for these properties.

Fixes #4950

Some folks didn't realize these existed and asked us to add them. By
using them in examples, hopefully folks will discover them more easily.

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

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.

The appbar now scrolls off screen when you scroll the page far enough.

BUG=https://github.com/flutter/flutter/issues/4410

* Change how navigator prevents redundant operations

Instead of requiring transactions, we now cancel all active pointers that are
interacting with the navigator and absorb future pointers until we get a chance
to build. This approach isn't perfect (e.g., events that trigger off the
cancelled pointers could still interact with the navigator), but it should be
better than the current transaction-based approach.

Fixes #4716

* Remove openTransaction

* test

* fixup

Anywhere that accepted IconData now accepts either an Icon or an
ImageIcon.

Places that used to take an IconData in an `icon` argument, notably
IconButton and DrawerItem, now take a Widget in that slot. You can wrap
the value that used to be passed in in an Icon constructor to get the
same result.

Icon itself now takes the icon as a positional argument, for brevity.

ThemeData now has an iconTheme as well as a primaryIconTheme, the same
way it has had a textTheme and primaryTextTheme for a while.

IconTheme.of() always returns a value now (though that value itself may
have nulls in it). It defaults to the ThemeData.iconTheme.

IconThemeData.fallback() is a new method that returns an icon theme data
structure with all fields filled in.

IconTheme.merge() is a new constructor that takes a context and creates
a widget that mixes in the new values with the inherited values.

Most places that introduced an IconTheme widget now use IconTheme.merge.

IconThemeData.merge and IconThemeData.copyWith act in a way analogous to
the similarly-named members of TextStyle.

ImageIcon is introduced. It acts like Icon but takes an ImageProvider
instead of an IconData.

Also: Fix the analyzer to actually check the stocks app.

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

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