This copies VM's convert implementation in dart2wasm's default and JSCM
targets, to be able to optimize the implementation based on these
targets' typed data and string implementation classes.
An example optimization that specializes UTF-8 decoder to `_U8List` can
be seen in [1], which improves a real-world benchmark (extracted from
internal) from 220ms to 90ms. (-59%)
Another optimization is we will be using the browser's UTF8 and JSON
decoders in JSCM convert patch.
VM's convert patch is moved from vm_shared to vm, as it's no longer used
outside of VM.
[1]: https://dart-review.googlesource.com/c/sdk/+/330121
Tested: existing tests.
Change-Id: I981070615a106e4f356ed8b292a29ec950bd4d97
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/330781
Reviewed-by: Jackson Gardner <jacksongardner@google.com>
Commit-Queue: Ömer Ağacan <omersa@google.com>
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