Refactor `BuildOp.onBuilt`

[daohoangson]

• Rename BuildOp.onTreeFlattening -> .onFlattening
• Rename BuildOp.onWidgets -> .onBuilt
• Rename WidgetPlaceholder.localName -> .debugLabel
• Remove listOrNull

[github-actions[bot]]

Demo app

e5650fa67d878c14fd5904cbd8451da4fbd48e02 APKs:

• app-armeabi-v7a-release.apk
• app-arm64-v8a-release.apk
• app-x86_64-release.apk
• Debug APK via https://appetize.io

Web build has been deployed to https://example-jya6jevzq-flutter-widget-from-html.vercel.app. Quick links:

• supported/tags.html
• Hello World
• Hello World (core)
• Wordpress

[codecov[bot]]

Codecov Report

Patch coverage: 99.59% and project coverage change: -0.03 :warning:

Comparison is base (38452fd) 98.84% compared to head (e5650fa) 98.81%.

Additional details and impacted files

@@                     Coverage Diff                     @@
##           refactor/build_metadata     #748      +/-   ##
===========================================================
- Coverage                    98.84%   98.81%   -0.03%     
===========================================================
  Files                           63       63              
  Lines                         3891     3890       -1     
===========================================================
- Hits                          3846     3844       -2     
- Misses                          45       46       +1     

Impacted Files	Coverage Δ
packages/core/lib/src/core_data.dart	100.00% <ø> (ø)
...ackages/core/lib/src/internal/margin_vertical.dart	87.50% <0.00%> (-5.84%)	:arrow_down:
...re/lib/src/internal/ops/style_text_decoration.dart	100.00% <ø> (ø)
packages/core/lib/src/internal/ops/tag_a.dart	100.00% <ø> (ø)
packages/core/lib/src/internal/ops/tag_q.dart	100.00% <ø> (ø)
packages/core/lib/src/internal/ops/tag_ruby.dart	100.00% <ø> (ø)
packages/core/lib/src/core_helpers.dart	100.00% <100.00%> (ø)
packages/core/lib/src/core_widget_factory.dart	100.00% <100.00%> (ø)
packages/core/lib/src/data/build_bits.dart	100.00% <100.00%> (ø)
packages/core/lib/src/internal/builder.dart	100.00% <100.00%> (ø)
... and 20 more

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