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.

Support exposing the InspectorService over the Flutterservice extension protocol as well as the observatory protocol. (#15876)

* Support exposing the InspectorService over the Flutter
service extension protocol as well as the observatory protocol.

We will probably remove most of the observatory protocol support once a
couple versions of the Flutter IntelliJ plugin have shipped that use the
Flutter service extension protocol. The only reason to continue supporting
the observatory protocol is it will allow using the inspector when paused
at a breakpoint.

* Fix the confusing-zero case with NestedScrollView.

* Update mock_canvas.dart

* Update tabs_demo.dart

* more tweaks

* Inspector robustness fixes.
Avoid cases where setState was called on disposed object.
Avoid cases where the inspector wouldn't render as it was called
after the first frame rendered and no other frames were rendered.

This attempts to apply some of what we learnt from UX studies, namely
that people wonder how to add multiple children to widget that take
one child.

Small code changes as part of moving the framework SDK forward to a 2.0 dev version.

Fixes https://github.com/flutter/flutter/issues/10695

...instead of the engine.

This allows the scheduler library to depend on the services library
and the painting library to depend on the scheduler library without
the services library having to depend on the scheduler library.

While I was at it I also cleaned up some of the binding logic: the
licenses logic can now be overridden (and the test library does so),
and the image cache can now be overridden as well.

Separate focus management by build owner

Currently the focus manager is held by the singleton WidgetBinding.
This change places a focus manager in each build owner object,
which allows clients to run offscreen pipelines without disturbing
focus management for the main UI.

* Add framework-side support for system text scale factor.

* Rolling engine to e3404b81a53ba3180c7623a6f2190ebb28518f30
  Additional changes rolled in with engine change:
    libtxt: implementation of GetRectsForRange that processes a line at a time - e3404b8
    Provide an entropy source to the Dart engine (#4161) - e1aa867
    libtxt: search for fallback fonts that can match emoji and CJK characters - 8061df1
    Roll skia to e4679fa06a. (#4157) - 267e7a8
    Update buildroot to 53fea9aebbcc39c6522731471a1a45960ee0685e (#4160) - 02ea7ae
    Revert engine Dart roll. (#4158) - 14aab33
    Add support for system text scale factor. (#4124) - b2a7f4b
    Include _http into sky_engine libraries for analyzer (#4154) - b930f10
    libtxt: Remove postprocess_line and improve tracking of X offsets - 86f95f0
    libtxt: remove redundant line_widths (#4152) - 14bf515
    Roll dart to ade37f931e90b0fdb8fe16d6bf6f089545da55b6 (#4151) - 6f1264f

Bare bones widget inspector support.

Toggle the widget inspector from the flutter tool by pressing 'i'.
When the widget inspector is select mode:
Pointer down to to inspect a widget.
Pointer click to finalize selection of a widget. You can now interact
with the application as you normally would but with the inspected widget
highlighted.
Click the inspect icon in bottom left corner of screen to reactivate
select mode.

It was previously possible for event dispatch to occurr during the
brief window where the tree had been marked dirty but before it had
been relaid out by reassemble, which would cause assertions to fire if
someone did a hot reload while touching the device.

...and break the navigator<->routes circular dependency.

This is a step towards fixing https://github.com/flutter/flutter/issues/9577 but doesn't yet do so.

This prevents some of our mixins from being subclassed.

Also, move mixins to using 'extends' instead of 'implements' for
future compatibility with Dart changes.

Also, rename a class that had Mixin in the name but was not a mixin.

Fixes https://github.com/flutter/flutter/issues/10399

* Rolls the engine to 75c74dc463d56e17be10315cfde409010fd8f90b.
* Adds framework support for the `AppLifecycleState.inactive` and `AppLifecycleState.suspending` states.

What are the new states?
------------------------
* `AppLifecycleState.inactive` is emitted on iOS only and corresponds to iOS's foreground inactive state. Current iOS state transitions are:
`resumed` <--> `inactive` <--> `paused`

* `AppLifecycleState.suspending` is currently emitted on Android only and corresponds to the
transition to Android's stopped state. Current Android state transitions are:
`resumed` <--> `paused` --> `suspending` --> `resumed`

These transitions may change in future.

This is a breaking change on iOS
--------------------------------

This changes the semantics of the `AppLifecycleState.paused` state on
iOS. The behaviour associated with this state is unchanged on Android.

For background on iOS application states see the state transition
diagram in the [App Programming Guide for iOS](https://developer.apple.com/library/content/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/TheAppLifeCycle/TheAppLifeCycle.html#//apple_ref/doc/uid/TP40007072-CH2-SW6).

On iOS, prior to this change, `AppLifecycleState.paused` corresponded to
a transition to the _foreground inactive_ state. It now corresponds to a
transition to the _background state_. The newly-added
`AppLifecycleState.inactive` state now corresponds to entering the
_foreground inactive_ state, which (currently) has no exact analogue on
Android.

Briefly, the _foreground inactive_ state is the state entered when
switching from an app to the app switcher, receiving a phone call, or
responding to a TouchID request. Apps are permitted to continue
animating/updating views in this state if desired. From the _foreground
inactive_ state, the app may transition back to _active_ (e.g., entering
the app switcher then resuming the app), or to the _background_ state
(e.g., switching to the home screen or another app).

What to change
--------------
If your app does not handle the `AppLifecycleState.paused` state in a
`WidgetsBindingObserver.didChangeAppLifecycleState` implementation, no
changes are required.

If you do handle `AppLifecycleState.paused`, you may additionally wish
to also handle `AppLifecycleState.inactive`. For example, games should
probably pause on entering the app switcher rather than wait to be
backgrounded.

More details
------------
For background on Android application states see the state transition
diagram in the [Android Activity Lifecycle docs](https://developer.android.com/guide/components/activities/activity-lifecycle.html#alc).

This splits the frame pipeline into two, beginFrame and drawFrame.

As part of making this change I added some debugging hooks that helped
debug the issues that came up:

 * I added debugPrintScheduleFrameStacks which prints a stack whenever
   a frame is actually scheduled, so you can see why frames are being
   scheduled.

 * I added some toString output to EditableText and RawKeyboardListener.

 * I added a scheduler_tester.dart library for scheduler library tests.

 * I changed the test framework to flush microtasks before pumping.

 * Some asserts that had the old string literal form were replaced by
   asserts with messages.

I also fixed a few subtle bugs that this uncovered:

 * setState() now calls `ensureVisualUpdate`, rather than
   `scheduleFrame`. This means that calling it from an
   AnimationController callback does not actually schedule an extra
   redundant frame as it used to.

 * I corrected some documentation.

We now attempt to explain what went wrong.

Fixes #9509

This is such a fundamental property of the system that we forgot to
document it.

Fixes #6818

We now have an explicit focus tree that we manage. Instead of using
GlobalKeys to manage focus, we use FocusNode and FocusScopeNode objects.
The FocusNode is Listenable and notifies when its focus state changes.

Focus notifications trigger by tree mutations are now delayed by one
frame, which is necessary to handle certain complex tree mutations. In
the common case of focus changes being triggered by user input, the
focus notificiation still arives in the same frame.

Fixes #7793.

Breaking change: removed deprecated methods of PlatformMessages, leaving only binary messaging there. All other use of platform communication now goes through PlatformMessageChannel and PlatformMethodChannels. Retained use of String and JSON codecs for now.

Companion engine PR: flutter/engine#3482

Covers lib/ in package:flutter.

...and fix bugs that the tests uncovered.

WRITE TEST FIND BUG

This allows us, for example, to wait for the slow mode banner to have
been removed from the screen before triggering a screen shot.