Enables pinch-to-zoom in InteractiveViewer on web via PointerScaleEvent.

* Implement trackpad gestures in framework

* Touch and Pan/Zoom pointers have separate IDs now

* Handle trackpad pointer device type

* Respect supportedDevices for pan/zoom events

* Update after rebase

* Fix check failures

* Avoid error with very short drags

* Address feedback

* Refactor drag event handler

* Address more feedback

* Add some missing punctuation

* PointerDeviceKind and ui.PointerChange forwards-compatibility

* Add link to trackpad gestures issue

* Add TODO

Remove references to `Window`, and switch usages to `PlatformDispatcher` or `SingletonFlutterWindow` (#69617)

* Remove references to dart:ui.Window, and point usages to PlatformDispatcher or SingletonFlutterWindow, as appropriate

* remove new test platform dispatchers

* Amend documentation

My yearly pass to fix spelling errors in the API dartdocs.

* PointerEvent.local* properties are calculated lazily; other properties are delegated to original.
* Transformed PointerEvent becomes a subclass of its original class.
* Unnamed constructors no longer accepts transform and origin.

Currently the framework uses fairly large "hit slop" values to disambiguate taps from drags/pans. This makes sense on touch devices where the interaction is not very precise, on mice however it can feel as if the UI is lagging. This is immediately noticeable on our infra dashboard, where it takes almost half of a grid square of drag before the actual drag kicks in.

One potential solution is to always use smaller constants depending on whether the interaction is mouse or touch based. The only reasonable choice is to use the pointer device kind and not target platform - same platform can have different input sources. This requires exposing the pointer device kind in a few new places in several of the gesture detectors, and using the enum to compute the correct hit slop from an expanded set of constants.

This almost works, however there are a few places (notably ListViews) which uses the touch hit slop as a default value in scroll physics. It does not seem like it will be easy to disambiguate a user provided scroll physics constant from the default and/or adjust it somehow - this might require significant changes to scroll physics which I have left out of this PR.

This PR does not adjust:

kTouchSlop used in scroll_physics.dart's minFlingDistance
kTouchSlop used in PrimaryPointerGestureRecognizer/LongPressGestureRecognizer

* Redesigns the interface between MouseTracker and RendererBinding&RenderView.
* Simplifies the structure of RenderMouseRegion.
* Extracts the common utility code between mouse_tracker_test and mouse_tracker_cursor_test.

* Add punctuation for unterminated sentences in doc comments

* Addressing review nit

This reverts commit 2ab50994.

This field is used to look-up the event in the platform
that resuted a given `PointerEvent`. This is currently only
used on Android, where the `embedderId` is set to be the
`motionEventId` for a given `MotionEvent`.

Roll engine to d0d6a4c2362d2ed478006bb3b01c34c0e96033b4

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

This converts Diagnosticable to be a mixin instead of an abstract class, so that it can be used to add diagnostics to classes which already have a base class.

It leaves in place the DiagnosticableMixin mixin, since there are some plugins that are still using it, and removing it would mean that those plugins wouldn't work with master branch Flutter anymore. DiagnosticableMixin will be removed once this mixin version of Diagnosticable makes its way to the stable branch.

This normalizes all of the "See also" blocks in comments so that they conform in style.

They all look like this now:

  /// See also:
  ///
  ///  * [MyFavoriteWidget], because it's cool.
  class Foo {}

I removed some useless "See also" blocks, and added commentary to ones that were just "bare" references.

This is my penance for adding so many bullets that are "-" instead of "*". :-)

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

* implicit-casts:false in flutter/lib/src/gestures

* address review comments

No code changes, just comments: I spell-checked all the comments in the repo.