Makes debugging semantics easier (e.g. which RenderObject has semantics, where is a SemanticNode seen in the semantic tree comming from, etc).

It's only printed if accessibility is turned on.

* 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).

* update the docs css style so the generated dartdoc matches the new flutter.io

* Change to 0.12.0 dartdoc

* Review comment

Move some Cupertino colors to a common file. 

Create a CupertinoTabBar widget to mimic iOS looks

...and rearrange a bunch of code so that all these arguments/members
are always in the same order.

* expose debugPaintBaselinesEnabled

* Update binding.dart

review comments

I added a bunch of sample code to the core Widget subclasses so that
people would see that you are supposed to always include the `key`
argument.

* Let run default to first profile

* fix

* review notes

I added some tests for the bug that I fixed.
I added docs for IconButton and AppBar.
I added some new constructors for FractionalOffset.

This reverts commit 4a920557.

Does not yet migrate hasIdeviceId since that results in a cascade of
breaking interface changes that's significant enough for a separate
patch.

* Revert "Test installation status when ideviceid is not installed (#10254)"

This reverts commit 0e5d4a87.

* Revert "Partial rollback of #10204 (#10256)"

This reverts commit b291bf5d.

Ensure that flutter doctor returns a partial installation status when
ideviceid is not installed.

This broke Fuchsia as they haven't updated to Dart 1.24 yet :(

Our emulator detection was based on a simple heuristic that was
failing for the Samsung Galaxy S8. Any heuristic is flawed since
Android devices can report whatever they want to adb, but this
change attempts to tighten the detection by listing known models
(by their ro.hardware property). Again, these values could be
spoofed by emulator system images, but it's less likely to be
an issue than with our previous (and fall-back) heuristic.

Fixes #10203
Related: #10248

This helps to debug encoding issues that are believed to only happen for certain locales (see https://github.com/flutter/flutter/issues/10198 fo example).

DO NOT SUBMIT until Dart SDK has been bumped (https://github.com/flutter/flutter/pull/10110) as this depends on new API.