Window.viewInsets is the set of window-relative insets that describe the
area of the window that an application may want to treat as effectively
reducing the size of the content. Typically this is due to system UI
that fully obscures underlying content, such as the keyboard.

This area differs from padding in that padding is the set of insets that
describe the area of the window that may be partially (or fully)
obscured by system UI or physical intrusions into the view area (e.g.
iPhone X sensor housing, status bar, or the iPhone X home indicator
widget).

This patch does not yet enable the iOS bottom edge safe area. Once the
framework has been updated to use viewInsets for bottom-edge occlusions
(today, the keyboard), the bottom safe area will be enabled and
framework patches that depend on it, landed.

* systems/settings channel split

* merge textScaleFactor and alwaysUse24HourFormat into flutter/settings channel

* add debugOverrideAlwaysUse24HourFormat

* implement textScaleFactor on iOS

* address comments

* remove debugOverrideAlwaysUse24HourFormat

* clang-format

Adds support for system text scale factor, including hooks for Android system settings changes.  iOS hooks will be added in another PR.

* fix (with squashed debug stuff)

* clean up

* preserve Window callback zones

Run Window callbacks in the zone they are registered in. This is consistent with how other native API work, such as `scheduleMicrotask`, `Timer`, and `dart:io`. This also enables the developers to use the `Zone` API to capture and log unhandled Dart errors.

* refactor wrapping

* new line

* fewer if checks; group getters/setters/fields

* inline _invokeOnPointerDataPacket

**This is a breaking change on iOS**

Previously, the `paused` state was entered when the application resigned
active status. `inactive` now maps to this status. `paused` now maps to
an app that has been backgrounded.

`inactive` is currently emitted on iOS only and corresponds to iOS's
foreground inactive state.

Current state transitions are:
`resumed` <--> `inactive` <--> `paused`

suspending is currently emitted on Android only and corresponds to the
transition to Android's stopped state.

Current state transitions are:
`resumed` <--> `paused` --> `suspending` --> `resumed`

These transitions may change in future.

See https://github.com/flutter/flutter/issues/7626

This name is consistent with how the other `dart:` libraries are named now.

We no longer use mojom to transport messages. We still use the Mojo EDK
to spin the event loop, however.

Previously, these signals were sent over Mojo. Now we send them through
platform messages.

Add plunbing for Host-to-Dart platform messages

We'll use the plumbing in this direction for things like key events.

The PlatformPlugin is the first of the built-in plugins that will replace the
mojom platform services.

This transport uses the Dart and JNI APIs directly instead of indirecting
through Mojo.

This patch prepares us to switch sending semantics information over mojom to
using dart:ui directly. Currently the recorded information is dropped on the
floor, but that will improve in future patches.

Long live pointer data packets.

Rather than using mojom to encode pointer data, we now encode and decode it
manually. A future patch will remove the mojom codepath once the framework is
updated.

This patch makes three API changes:

 * The dart:ui library now always communicates in physical pixels. The
   framework is responsible for converting to whatever logical coordinate
   system it wishes to use.
 * The textBaselien property is now on TextStyle rather than ParagraphStyle,
   which will let us choose which baseline to use on a per-span basis rather
   than on a per-paragraph basis.
 * This patch also removes the old MojoServices function endpoints.

Fixes https://github.com/flutter/flutter/issues/3779
Fixes https://github.com/flutter/flutter/issues/1360

Also, rename SkyViewClient to RuntimeDelegate. These names are more
sensible.

This patch also cleans up the RuntimeDelegate a bit, for example by
removing support for flushing real-time events, which aren't used.

This change disentangles the build for the snapshotter from the main engine
build. Also, we now have all the Dart files for dart:ui in the same directory.

This would have answered a question we received on flutter-dev.

Fixes https://github.com/flutter/flutter/issues/3517

Previously we would hold the layer tree until the we returned from Dart. Now we
send the layer tree to the rasterizer thread immediately.

Fixes #212

The layout protocol in Mozart changed. Now we need to pipe a scene version
through the system to the compositor.

Adds or augments the documentation of various objects in dart:ui.

Removes the ignored argument to SceneBuilder.

Hide the default constructor on Gradient.

Add a call to the ImageShader constructor.

Reorder some members so that private versions come after public versions
so that docs will come before both.

Also, add some dartdocs for AppLifecycleState values.

And trivial docs changes.

This adds some dartdocs to dart:ui, specifically for Window and friends.

We now respect the "route" field in Intents to load a route other than '/'.
Also, use popRoute rather than events to indicate that the framework has asked
us to go back.

With this change we no longer need engine/core/events and its IDL bindings.