[dart2wasm] Optimize UTF8 decoder when decoding a native array

Intercept all `Utf8Decoder.convert` calls to check the input type and
call the right UTF8 decoder function.

This currently only optimizes `U8List` parsing, and only the fastest
path where the input is an ASCII string. Follow-up CLs will optimize JS
typed array parsing by calling browser's UTF8 decoder and the slow paths
of non-ASCII and malformed buffers.

Results from an internal benchmark extracted from a real use case:

Before:

    Parse(RunTime):     330,500 us.

After:

    Parse(RunTime):     90,310 us.  (-72%)

dart2js -O4 results of the same benchmark:

    Parse(RunTime):     234,250 us.

Change-Id: I470697a9eb516e1e031e9b865ddb29e8f25569bf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/331187
Commit-Queue: Ömer Ağacan <omersa@google.com>
Reviewed-by: Jackson Gardner <jacksongardner@google.com>