Update Dart flute WasmGC workload

This updates the Dart flute benchmark. Apart from using a newer
Dart2Wasm compiler this brings two important changes:

  * The compiler & standard library will now use JavaScript strings as
    it's only string implementation. It takes advantage of `js-string`
    builtins if available.

    We anticipate all browsers adopting the `js-string` builtin
    proposal and optimize the builtin methods.

    NOTE: We currently don't *require* the `js-string` builtin to be
    available. We polyfill the imported methods and we import string
    constants from the mjs file. The main reason for this is that
    JavaScriptCore doesn't support `js-string` builtin yet (filed [0]).
    Once JSC supports the proposal we want to require it as it allows
    using the much more efficient string constant import mechanism
    (where string constants are utf8-encoded in the wasm module instead
    of importing from JS file)

    [0] https://bugs.webkit.org/show_bug.cgi?id=287402

    (Before the compiler used 3 implementations: `{One,Two}ByteString`
    and `JSString` and copied the strings across the WasmGC<->JS
    boundary. But now that we get more performant JS strings we prefer
    using only JS strings due to avoiding the need to copy them across
    the boundary, working with JS RegExp, ...)

  * The flute benchmark is now compiled in `-O2` mode that ensures Dart
    soundness. It will perform runtime type checks as required by the
    Dart language as well as throw catchable exceptions on out-of-bounds
    accesses.

    (Before the benchmark was compiled in `-O4` mode which is unsound
    Dart, it didn't perform the type checks required by the Dart language
    specification and didn't perform index checks).

    We intend to switch Flutter web to also use `-O2` mode.

Author: mkustermann

Dart/build.log

@@ -1,5 +1,5 @@
-Built on 2025-01-28 15:34:10+01:00
+Built on 2025-03-06 09:52:17+01:00
 Cloning into 'wasm_gc_benchmarks'...
-cf32ca4 Recompile all benchmarks
+f80892d Update Dart SDK, switch wasm to -O2, recompile performance benchmarks
 Copying files from wasm_gc_benchmarks/ into build/
 Build success