[devtools_extensions] build_and_copy ships ~14 MB of assets the extension never loads (CanvasKit variants + un-tree-shaken icon font) #9897

Summary

dart run devtools_extensions build_and_copy produces extension builds that are CDN-configured (the default for flutter build web), yet copies the entire local canvaskit/ directory — every engine variant plus debug symbols — into extension/devtools/build/, and hardcodes --no-tree-shake-icons. The result is ~14 MB of published assets per package that the extension never fetches at runtime.

Since the extension build is published inside each package's pub archive, every package that ships a DevTools extension carries this weight, and every consumer downloads it on pub get.

Evidence

My package (kaisel): archive was 15 MB. After pruning build/canvaskit/ and rebuilding with icon tree-shaking, it is 1 MB, with no runtime behavior
change:

• flutter_bootstrap.js in the build references gstatic.com/flutter-canvaskit, so the local canvaskit/ files (canvaskit.wasm 6.9 MB, chromium/ 5.5 MB, skwasm_heavy 4.9 MB, experimental_webparagraph/ 3.9 MB, skwasm 3.4 MB, plus ~7 MB of .js.symbols) are never requested.
• The build is compileTarget: dart2js, so the skwasm* variants are unreachable regardless of CDN configuration.
• Rebuilding without --no-tree-shake-icons succeeds — the compiler's own verification passed (it fails the build on non-const IconData) — and MaterialIcons went from 1,645,184 bytes to 9,040 bytes (99.5%).

Proposal

1. Prune build/canvaskit/ in the copy step when the build is CDN-configured (the default). At minimum, always prune the skwasm* / experimental variants for dart2js builds, where they are unreachable. If an author builds with --no-web-resources-cdn (e.g. for offline use — related: DevTools fails to load with no internet connection #4721), keep canvaskit.wasm + chromium/canvaskit.wasm and drop only the unreachable variants and .js.symbols.
2. Expose a --tree-shake-icons passthrough (keeping today's --no-tree-shake-icons as the default if desired). Tree-shaking is verified at compile time — it fails loudly when an extension constructs IconData dynamically — so extensions that pass the check could opt into the ~1.6 MB saving safely.

Environment: devtools_extensions 0.5.1, Flutter 3.44.2 (Dart 3.12.2), macOS.