Refactor TestPlatformDispatcher - #192747

…n tests

Remove the per-view TestPlatformDispatcher indirection (_forView,
_testValuesOwner, _testValues, _sharingTestValues). With only this PR
applied there is one dispatcher per binding, so _testValues resolved to
"this" everywhere. The indirection exists to serve a later PR and
belongs there.

Keep a custom view that shadows an engine view from rebuilding its
TestFlutterView on every refresh, which discarded the test values set
on it.

Route the teardown unfocus through notifyViewFocusChanged rather than
the wrapped dispatcher's callback field.

Tests: the custom-view test now refreshes the registry through
notifyMetricsChanged instead of calling back the callback it just
registered, which refreshed nothing. The chaining tests read the
previous callback back through the getter, which is what the symmetry
is for. New tests cover the focus bookkeeping, the refresh-before-notify
ordering, and shadowing.

Docs: drop a dangling [debugViewMetricsOverrides] reference and correct
resetWindowingOwner, which does not destroy the previous owner's
windows.

BEHAVIOUR CHANGE: fixes a reproduced view leak in removeTestView.

Window controllers hold the TestFlutterView wrapper that addTestView
returns, and _destroyTestWindowController passes that wrapper to
removeTestView. The staleness check guarding the fallback refresh only
compared against the backing view, so it was always false for that
caller. Normally harmless, because the metrics change refreshes the
registry anyway. But destroying a controller from inside a
metrics-changed notification makes that refresh reentrant, and the
guard drops it by design, so the fallback was the only thing left and
it never fired. The destroyed view then stayed in views and view(id:)
permanently.

Reproduced with a throwaway test before fixing: after a reentrant
destroy the registry still reported the destroyed view. removeTestView
now accepts the wrapper as well as the backing view. A regression test
covers it, mutation-verified in both directions.

The fix is in removeTestView rather than at the call site because the
backing view is not reachable from the controller and _view is private
to window.dart.

Boundary correction, per the repository owner's decision:

TestFlutterView.render and the physicalConstraints getter are reverted
to master's form. Both carried fallbacks that belong to the piece that
introduces debug view metric overrides:

- render used `size ?? _physicalSize`. Provably inert here: the only
  caller, RenderView.compositeFrame at rendering/view.dart:360, always
  passes a non-null size, and no other caller omits it. Its comment
  also forward-referenced debug overrides, which do not exist in this
  piece.
- physicalConstraints fell back through _physicalSize. Reachable only
  by calling resetPhysicalConstraints() without resetPhysicalSize(),
  where it changed behaviour versus master and left
  resetPhysicalConstraints not fully resetting. No test covered it.

Test coverage, both gaps confirmed by mutation first:

- clearAllTestValues dropping custom views was unpinned. Deleting
  _customViews.clear() and the refresh left all 714 tests green.
- The notify: false argument in _TestFlutterView.register was
  unpinned. Removing it left all 94 windowing tests green.

Test hygiene: three tests replaced the shared dispatcher's
onMetricsChanged or onViewFocusChange without restoring it. One never
restored it at all; two restored it mid-test, which an earlier failing
expect would skip. All three now restore through addTearDown.

Gate: dart analyze --fatal-infos clean, dart format 0 changed,
flutter_test 715 passing, flutter test/widgets 7327 passing.

Findings rejected with evidence, for the record:

- "removeTestView leaves a dangling _currentlyFocusedViewId, and
  clearAllTestValues does not clear focus state." Reproduced with a
  probe: nothing throws and the state converges to null. It matches the
  documented contract that currentlyFocusedViewIdTestValue records the
  last focus event rather than querying a live view. The
  clearAllTestValues half is pre-existing master behaviour, untouched
  by this PR.

- "Add @VisibleForTesting to createWindowingOwner." Rejected: the
  method is invoked by WidgetsBinding.initInstances on every production
  app start, so it is not test-only. Adding it was also tried and does
  not produce the diagnostic that was claimed to justify it.

- "The constraints != null fallback in _TestFlutterView is inert
  scaffolding." It is verbatim master code, untouched by this PR.

Noted but deliberately NOT fixed here, as unrelated to this split:
TestPlatformDispatcher's `set onError` is a no-op. It evaluates
_platformDispatcher.onError as a statement and discards it, never
assigning value. That is pre-existing on master and untouched by this
PR; it wants its own change.

…fter teardown

…stWidgetsFlutterBinding

…est-platform-dispatcher

…patcher

# Conflicts:
#	packages/flutter/test/widgets/windowing_test.dart