Adds macOS support for `flutter create`:
- Currently it is behind a hidden flag.
- Adds a TargetPlatform workaround to lib/main.dart in the standard app template when enabled.
- Supports `app` and `plugin`; `module` support doesn't yet exist for macOS in general.

This will eliminate the need to use FDE's examples as templates on macOS. The templates are based on the current state of FDE's examples, with templating support added (and with adoption of the new application delegate in the app, which hadn't been done yet in FDE, eliminating some boilerplate from the template).

Fixes #30703 

Rather than have a monolithic .gitignore covering all platforms in the
app template, have each platform folder contain its own rules.

Fixes #40793

Started reading stored channel messages from ui.channelBuffers when handlers get set.

* Allow sending messages from the platform to the framework

* Add ability to send messages from the plugin to the platform.

Also adds [PluginEventChannel] which is the plugin counterpart to
EventChannel.

Fixes https://github.com/flutter/flutter/issues/39981

* fix analyzer errors

* Enhance doc comments

* Remove dead code in test

* Fixed Selectable text align is broken

1cada0a6d Roll src/third_party/skia 6344cbd1df76..18531cde2ab7 (2 commits) (flutter/engine#12358) (#40842)


git@github.com:flutter/engine.git/compare/bf3af4d157cc...1cada0a6dca6

git log bf3af4d157cc..1cada0a6dca6 --no-merges --oneline
2019-09-19 skia-flutter-autoroll@skia.org Roll src/third_party/skia 6344cbd1df76..18531cde2ab7 (2 commits) (flutter/engine#12358)


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git@github.com:flutter/engine.git/compare/dd081bea0389...bf3af4d157cc

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2019-09-19 skia-flutter-autoroll@skia.org Roll src/third_party/skia e2a903e18b59..6344cbd1df76 (3 commits) (flutter/engine#12357)
2019-09-19 bkonyi@google.com Roll src/third_party/dart cb80ea7ba9..ea969c358e (52 commits)


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git@github.com:flutter/engine.git/compare/7fb14000fddf...dd081bea0389

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2019-09-19 dnfield@google.com skip flaky test (flutter/engine#12355)


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git@github.com:flutter/engine.git/compare/92afbfdf544d...7fb14000fddf

git log 92afbfdf544d..7fb14000fddf --no-merges --oneline
2019-09-19 dnfield@google.com java lints (flutter/engine#12354)
2019-09-19 iska.kaushik@gmail.com [flutter_runner] Port vulkan surface changes (flutter/engine#12350)


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92afbfdf5 Roll src/third_party/skia f19f9caab2b4..e2a903e18b59 (1 commits) (flutter/engine#12351) (#40816)


git@github.com:flutter/engine.git/compare/eed16d34bd19...92afbfdf544d

git log eed16d34bd19..92afbfdf544d --no-merges --oneline
2019-09-19 skia-flutter-autoroll@skia.org Roll src/third_party/skia f19f9caab2b4..e2a903e18b59 (1 commits) (flutter/engine#12351)


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When the plugin list changes, iOS pods are invalidated, but that was
never wired up for macOS.

Should fix #39327

git@github.com:flutter/engine.git/compare/33ad6757e883...eed16d34bd19

git log 33ad6757e883..eed16d34bd19 --no-merges --oneline
2019-09-19 iska.kaushik@gmail.com [flutter_runner] Explicitly set |trace_skia| to false (flutter/engine#12349)
2019-09-19 iska.kaushik@gmail.com [flutter_runner] Port the new compilation trace from topaz (flutter/engine#12348)
2019-09-19 iska.kaushik@gmail.com [flutter_runner][async] Migrate dart/flutter to new async-loop APIs (flutter/engine#12347)
2019-09-19 iska.kaushik@gmail.com [flutter_runner] Move from runner context to component context (flutter/engine#12346)


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* Use separate isolate for image loading. Use TransferableTypedData to const-cost receive bytes from that isolate.

git@github.com:flutter/engine.git/compare/7f7250f47636...33ad6757e883

git log 7f7250f47636..33ad6757e883 --no-merges --oneline
2019-09-19 iska.kaushik@gmail.com [flutter_runner] Port over the tuning advice (flutter/engine#12345)
2019-09-19 iska.kaushik@gmail.com [flutter_runner] Remove usages of shared snapshots from CC sources (flutter/engine#12343)


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git@github.com:flutter/engine.git/compare/00a12c411f81...7f7250f47636

git log 00a12c411f81..7f7250f47636 --no-merges --oneline
2019-09-18 iska.kaushik@gmail.com [flutter_runner] Do not use prebuilts just yet (flutter/engine#12340)


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git@github.com:flutter/engine.git/compare/d3c60acef82c...00a12c411f81

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2019-09-18 dnfield@google.com Add missing CL, fix targets for Fuchsia (flutter/engine#12338)


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git@github.com:flutter/engine.git/compare/00f23b2e7356...d3c60acef82c

git log 00f23b2e7356..d3c60acef82c --no-merges --oneline
2019-09-18 ditman@gmail.com More controls for Screenshot tests (for web) (flutter/engine#12284)
2019-09-18 skia-flutter-autoroll@skia.org Roll src/third_party/skia 8391bac5c6f0..f19f9caab2b4 (8 commits) (flutter/engine#12341)


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* Simplified the serialization/deserialization logic of the Descendant/Ancestor matchers

Before this change, having an Android app depend on a plugin that has no android implementation resulted in a Gradle build failure.

This scenario is likely to become more common if we're enabling federated plugins, as the package implementing just the desktop implementation of a plugin won't have an Android implementation.

This changes the Gradle plugin to not try to build any plugins that doesn't have an android/build.gradle file.

git@github.com:flutter/engine.git/compare/113be249ddcf...00f23b2e7356

git log 113be249ddcf..00f23b2e7356 --no-merges --oneline
2019-09-18 tamird@google.com [fuchsia] add fuchsia.netstack.Netstack (flutter/engine#12325)
2019-09-18 skia-flutter-autoroll@skia.org Roll src/third_party/skia dd0dff1c35d2..8391bac5c6f0 (8 commits) (flutter/engine#12337)
2019-09-18 ditman@gmail.com Ensure DRRects without corners also draw. (flutter/engine#12330)


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git@github.com:flutter/engine.git/compare/2289072724ff...113be249ddcf

git log 2289072724ff..113be249ddcf --no-merges --oneline
2019-09-18 amirh@users.noreply.github.com Update ExternalViewEmbedder class comment. (flutter/engine#11063)


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Added docs examples of getting the string from a TextField

The transition_perf test is no longer flaky.

228907272 Roll src/third_party/skia d545bfbb94ca..dd0dff1c35d2 (25 commits) (flutter/engine#12334) (#40761)


git@github.com:flutter/engine.git/compare/fce8be3a68c0...2289072724ff

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2019-09-18 skia-flutter-autoroll@skia.org Roll src/third_party/skia d545bfbb94ca..dd0dff1c35d2 (25 commits) (flutter/engine#12334)


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git@github.com:flutter/engine.git/compare/36be89da497b...fce8be3a68c0

git log 36be89da497b..fce8be3a68c0 --no-merges --oneline
2019-09-18 bkonyi@google.com Roll src/third_party/dart f22f62c85d..cb80ea7ba9 (8 commits)


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git@github.com:flutter/engine.git/compare/6a96417416b4...36be89da497b

git log 6a96417416b4..36be89da497b --no-merges --oneline
2019-09-17 matthew-carroll@users.noreply.github.com Added javadoc comments to FlutterActivity and FlutterFragmentActivity. (flutter/engine#12328)
2019-09-17 bkonyi@google.com Roll src/third_party/dart 7505b3a5f0..f22f62c85d (5 commits)
2019-09-17 iska.kaushik@gmail.com Revert "Provide dart vm initalize isolate callback so that children isolates belong to parent's isolate group. (#9888)" (flutter/engine#12327)
2019-09-17 ankitdave06@gmail.com [flutter] Remove old A11y API's. (flutter/engine#12308)


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git@github.com:flutter/engine.git/compare/d1692d4cc703...6a96417416b4

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2019-09-17 skia-flutter-autoroll@skia.org Roll fuchsia/sdk/core/linux-amd64 from RRgw-... to F-g18... (flutter/engine#12326)
2019-09-17 chinmaygarde@google.com Account for root surface transformation on the surfaces managed by the external view embedder. (flutter/engine#11384)
2019-09-17 matthew-carroll@users.noreply.github.com Introduce FlutterFragmentActivity (flutter/engine#12305)
2019-09-17 chinmaygarde@google.com Shuffle test order and repeat test runs once. (flutter/engine#12275)


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git@github.com:flutter/engine.git/compare/63873d9f421f...d1692d4cc703

git log 63873d9f421f..d1692d4cc703 --no-merges --oneline
2019-09-17 hterkelsen@users.noreply.github.com Update canvaskit backend (flutter/engine#12318)
2019-09-17 mouad.debbar@gmail.com README for the felt tool (flutter/engine#12323)
2019-09-17 jason-simmons@users.noreply.github.com Fix continuous event polling in the GLFW event loop (flutter/engine#12320)
2019-09-17 15365765+rafern@users.noreply.github.com Tests for #11283 (flutter/engine#12322)
2019-09-17 ditman@gmail.com Improve check to render (or not) a DRRect when inner falls outside of outer on RecordingCanvas (flutter/engine#12229)
2019-09-17 bkonyi@google.com Roll src/third_party/dart dd1969a43a..7505b3a5f0 (39 commits)
2019-09-17 30870216+gaaclarke@users.noreply.github.com Channel buffers (flutter/engine#12167)
2019-09-17 xster@google.com Make iOS FlutterViewController stop sending inactive/pause on app lifecycle events when not visible (flutter/engine#12128)
2019-09-17 stuartmorgan@google.com Adds PluginRegistry to the C++ client wrapper API (flutter/engine#12287)
2019-09-17 liyuqian@google.com Add "type" to getDisplayRefreshRate protocol (flutter/engine#12319)
2019-09-17 mouad.debbar@gmail.com Add a build command to felt (flutter/engine#12303)
2019-09-17 skia-flutter-autoroll@skia.org Roll src/third_party/skia df432d5efb70..d545bfbb94ca (1 commits) (flutter/engine#12316)
2019-09-17 skia-flutter-autoroll@skia.org Roll fuchsia/sdk/core/linux-amd64 from rwf0-... to RRgw-... (flutter/engine#12315)
2019-09-17 skia-flutter-autoroll@skia.org Roll src/third_party/skia f8486f2c5fb6..df432d5efb70 (1 commits) (flutter/engine#12313)
2019-09-17 skia-flutter-autoroll@skia.org Roll src/third_party/skia b47704b0bd34..f8486f2c5fb6 (2 commits) (flutter/engine#12312)
2019-09-16 jason-simmons@users.noreply.github.com Fix the declaration of setSystemGestureExclusionRects to match the PlatformMessageHandler interface (flutter/engine#12306)
2019-09-16 gw280@google.com Manage resource and onscreen contexts using separate IOSGLContext objects (flutter/engine#12277)
2019-09-16 goderbauer@google.com Cleanup in web_ui (flutter/engine#12307)
2019-09-16 30870216+gaaclarke@users.noreply.github.com Made flutter startup faster by allowing initialization to be parallelized (flutter/engine#10182)
2019-09-16 skia-flutter-autoroll@skia.org Roll src/third_party/skia c22498502cda..b47704b0bd34 (16 commits) (flutter/engine#12304)
2019-09-16 jonahwilliams@google.com Include firefox in check to quote font families (flutter/engine#12288)
2019-09-16 bkonyi@google.com Roll src/third_party/dart 7799f424f4..dd1969a43a (2 commits)


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Previously, focus nodes created in the tab scaffold were not being disposed of properly, causing possible memory leaks. Also, the builder wasn't being passed the right context so that the FocusScope.of operator inside of a builder would find the focus scope for the given tab (it was being passed the context of the overall tab scaffold).