Implement remaining methods of Firebase class on iOS

Wire in the device observatory port while initializing the Dart VM

Accessibility fixes

Make Mozart draw child views

Previously we weren't factoring in the layer's offset properly and we were
drawing the child views below the background layer.

- Correctly enable accessibility on startup if it's already enabled.
- Avoid using deprecated APIs.
- Map FORWARD/BACKWARD scrolling actions to up/down even when left/right
  is also possible.
- Clear the accessibility focus when removing a focused semantics node.
- Unregister listeners when view goes away.
- Call setWillNotDraw(false) if accessibility is enabled.
- Clean the handling of hover events.
- Work around an Android bug where the system focus rect isn't drawn
  the first time.

Serve the asset bundle directly out of an FLX zip file

Add a dynamic services target with a variant for the embedder and dylib each

* The embedder depends on dynamic:embedder
* The dylib depends on dynamic:dylib
* The embedder calls OnLoad and OnUnload callbacks that setup and teardown the dylib environment
* The dylib contains a thin library that services OnLoad, OnUnload and OnInvoke so that vendors dont have to do it themselves (and potentially mess it up)
* The vendor only has to implement the `FlutterServicePerform` method that takes a fully scoped handle
* This patch is a WIP till we get a stable Flutter ABI. The stuff in mojo/public is *NOT* stable

Fix-up tree-shaking entry points. Toward making missing entry points …

Update project.pbxproj

Fix sky_snapshot build to only link libgcc_s if on Linux

Pass -lgcc_s to sky_snapshot build.

libgcc_s is not a DT_NEEDED library; it gets loaded implicitly.
This change ensures that rpath is referenced when searching for
the so. This in turn gets the resulting sky_snapshot binary
closer to being able to run in Google's production environment.

Make the ios simulator build incremental wrt `pub get` - don't call it each time unless the pubspec file has changed.

Currently, during application startup we:
  * copy the FLX file to a Mojo pipe
  * write the Mojo pipe contents back to a temporary file on disk
  * unzip the FLX contents into cache storage
This contributes to startup latency and requires that we later clean up the
cache.

With this change, the assets will be extracted from the FLX archive
on demand with no writes to storage. Runtime cost should be minimal
given that most assets (except for the snapshot) are not compressed
in the archive.

Track whether accessibility is enabled.

If we send accessibility events when accessibility is disabled, we crash.

Make AccessibilityNodeInfos interactive.

Service vendors no longer specify the service entry point

Instead, the entry point is well known. The service name is passed as an argument to the service.

This lets you focus and unfocus a node.

Still doesn't seem to be enough to actually have the node exposed in
TalkBack. We probably need hover for that (that's next).

(Also fixes a leak for when a subtree has been disconnected; previously
we were only forgetting the top node of the subtree, not the
descendants. Thanks @krisgiesing for catching that.)

Hard to say if this actually works. We'll find out once we've filled
this out enough that you can walk the tree in Android's Talkback UI.

This exposes the actions on AccessibilityNodeInfo nodes.
It also tries to make the code that handles reloads more correct.

Roll Dart to 1.14.1

iOS: Dont strip release binaries

Dont keep the fake runner around as it fails validation when preparing release archives

Flutter<->Android accessibility bridge

This is another step towards enabling accessibility for Flutter on
Android. It exposes the semantics tree to Android's accessibility API
when accessibility is enabled.

It does not yet:
 - allow one to actually interact with the application via the
   accessibility API
 - expose the accessibility tree to touch exploration
 - implement the accessibility focus API

However, you can see the tree if you run uiautomatorviewer. It is there,
and it matches the UI. At least in Stocks. I didn't test anything else.

Pass the Mozart ViewHost into Dart