It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

By default BoxScrollView (and hence ListView, which is a subclass)
padding is the media padding along its scroll axis in order to avoid
placing list items within areas where user interaction should be
minimised -- e.g. under the status bar, or in and around the iPhone X
notch in landscape mode.

In cases where a list item should occupy the padding area, developers
should set the ListView padding to EdgeInsets.zero so as not to pick up
the default media padding. For widgets inside the drawer that should
avoid safe areas, developers can add a SafeArea widget.

Also includes minor doc fixes.

Fixes https://github.com/flutter/flutter/issues/9342

backgroundColor -> color
backgroundImage -> image
BackgroundImage -> DecorationImage

* Add a tile scrolling test to complex_layout app

* - Review comments and fix analyzer failures.

* Use ListView.builder pattern.

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

* upgrade to linter-0.1.30

* add prefer_is_empty lint
* add directives_ordering lint
* add no_adjacent_strings_in_list lint
* add no_duplicate_case_values lint
* add prefer_collection_literals lint
* add prefer_const_constructors lint
* add prefer_contains lint
* add prefer_initializing_formals lint
* add unnecessary_null_aware_assignments lint
* add unnecessary_null_in_if_null_operators lint

Nobody knew what a Block was.

Aggressively apply the const lint.

* use color.shadeXxx instead of color[Xxx]

* remove calls to .shade500 on MaterialColor

* remove calls to .shade200 on MaterialAccentColor

* fix test

We need to study these benchmark numbers more carefully. See #7937

In particular, we now use ListView in the about dialog.

This prepares us for a CustomScrollView that takes slivers.

The finder will only match the widget's ValueKey if both have identical
runtime types

This patch deploys ScrollView in a number of demos and manual tests.

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

* Added custom radii to RRect.

This is the first commit towads an implementation of
MergeableMaterial. It adds custom radii to RRect.

* Renamed RRect constructors and added BorderRadius.

BorderRadius is a class similar to EdgeInsets that lets you define
all rounded corners of a rounded rectangle easily.

And make Scrollbar work with LazyBlock.

And an about box to the Stocks sample app.

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.

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