* Move node implementation to Objective C

This changes AccessibilityBridge::Node (C++) to be AccessibilityNode (objc)
in order to have the nodes returned directly as ui accessibility elements.
This buys us a few things:

1) We can now return an element tree instead of a flat list, thus negating
   the need to maintain a global rect and global translation
2) We use less memory since we don't need to instantiate both nodes and
   their ensuing accessibility elements
3) accessibility elements can now implement the methods necessary to respond
   to events like user scrolling

https://github.com/flutter/flutter/issues/1664

This patch switches us to building Runner.app on the end-developer's machine,
which will eventually let them customize their app.

Hosting this script inside the repo will let us change the contents of the SDK
without having to synchronize a change on the infrastructure and in the repo.

These Dart_GetField calls show up on traces. This patch removes almost all of
them from the Canvas interface. We still have a few in drawAltas and
drawVertices, which will be removed in a later patch.

Fixes https://github.com/flutter/flutter/issues/3256

Remove the use of --simulator to indicate x64 on Android

Now we grab the underlying primative values before exiting the VM.

Partial fix for https://github.com/flutter/flutter/issues/2950

Instead of using a static Activity, we can use the one we get when we construct
the service.

After this patch, main_ios.mm now consumes Flutter.framework via public APIs.
This patch prepares us to move Flutter on iOS over to consuming
Flutter.framework as a binary artifact.

Fixes #https://github.com/flutter/flutter/issues/3689

The flutter tool is now using the observatory's HTTP-based interface

These are now offerred by the Flutter platform services (e.g., SystemChrome,
SystemSound, HapticFeedback).

When there are no pending timers, we should sit in glfwWaitEvents instead of
continuously calling glfwPollEvents. This reduces CPU utilization
significantly.

This reverts commit 5bc5c7b5.

This commit appears to have caused
https://github.com/flutter/flutter/issues/3658

These build definitions no longer work with the current version of FNL. Once
FNL is ready, we'll add new build definitions that work for it again.

Fix handling of backspace in text input.

All the clients of migrated to the new ParagraphConstraints API.

In the new ParagraphConstraints code path, we weren't converting through
LayoutUnit, so we weren't getting the right overflow behavior for extremely
large double values. This resulted in test/rendering/block_test.dart failing.

We don't need to use an explict version of the script snapshotter that is
packaged with the iOS SDK. Instead, we can rely on the flutter command having a
working snapshotter.

All the headers for Flutter.framework are now in framework/Headers and all the
source files for Flutter.framework are now in framework/Source. Previously it
was unclear that FlutterAppDelegate and main_ios.mm weren't part of
Flutter.framework. (They still build as part of the framework, but that's
something I'll fix in a later patch.)

This lets Skia run an optimization pass over the SkPicture before actually
issuing GL commands.

This patch begins to update the Paragraph interface to something sensible. It
introduces a ParagraphConstraints object that is passed to layout() to control
the layout of the paragraph. Once clients are migrated over, the argument will
become required.

Previously the AOT snapshot was packaged as a dynamic library (similar to the
scheme used on iOS).  This change eliminates the need for a native code
toolchain when building for AOT.

Instead of our own shadow copy.

Explicit import instructions in dart:jni

Report Flutter widgets to iOS accessibility API