Refactor `JSStringImpl` to use the js-string-builtins API[1].
When the module `WebAssembly.String` is not available we define a
"polyfill" with the previous imports, so this change is backwards
compatible.
Also updates some of the methods to use avoid multiple `this.length`
calls (which do an FFI call), and use unchecked getters when possible.
A new library `dart:_error_utils` is introduced for faster range and
index checks when the length (or max value) is known to be positive
(e.g. when it's the length of a string).
For now only `JSStringImpl` and `operator []` and `operator []=` of
`JSArrayImpl` are updated to use the new range and index checks. Rest of
the libraries will be updated separately.
[1]: https://github.com/WebAssembly/js-string-builtins
CoreLibraryReviewExempt: dart2wasm specific library change.
Change-Id: I9436def0cfe59c631f6f4e15ea06cc18a47a738e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/335043
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Ömer Ağacan <omersa@google.com>