Remove `toString()` overrides in `dart:ui`/`package:flutter` in profile/release mode on wasm/vm targets (#144763)

The flutter engine & framework can opt out of this optimization for
individual classes / class hierarchies via:

  * `@pragma(flutter:keep-to-string)`
  * `@pragma(flutter:keep-to-string-in-subtypes)`

Or by using the convenience constant `@keepToString` from `dart:ui`.

=> This aligns the build process more with g3 (which already does this)

Closes https://github.com/flutter/flutter/issues/52759