Use WidgetsBinding.instance.platformDispatcher in windowing instead of PlatformDispatcher.instance (#178799)

This is the recommended way to do this as it allows us to override the
platform dispatcher when that is necessary

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/eea45ac3b2beec7687febc378656726cd7e11af1

DEPS

diff --git a/DEPS b/DEPS
index 6c4e51d..2d9deed 100644
--- a/DEPS
+++ b/DEPS
@@ -232,7 +232,7 @@
 deps = {
   'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'a569828c56e38ffa56032a40148234270f8d56ab',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'd8fc22e25cbdadd6adfcbe968a24fb7f9ee103e8',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'eea45ac3b2beec7687febc378656726cd7e11af1',
 
   'flutter/engine/src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',

commits.json

diff --git a/commits.json b/commits.json
index d000914..cdcb78f 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
-  "flutter":"d8fc22e25cbdadd6adfcbe968a24fb7f9ee103e8",
+  "flutter":"eea45ac3b2beec7687febc378656726cd7e11af1",
   "flutter/engine/src/flutter/third_party/dart":"a569828c56e38ffa56032a40148234270f8d56ab"
 }
\ No newline at end of file