[dart2wasm] Remove short input threshold to fall back to native UTF-8 decoder

When decoding a JS array input as UTF-8 we currently fall back to the
decoder implemented in Dart when the input is small.

When benchmarked with `benchmark_harness`, for one byte array, I get
6.0us when decoded with the Dart decoder, and 5.8us when decoded with JS
`TextDecoder`.

So it looks like Dart decoder is never faster when the input is a JS
array. Remove the threshold and always call JS when the input is a JS
array.

(This threshold was copied from dart2js, where accessing JS data is less
expensive.)

Change-Id: I1a34fd1f38718e34a1ff7bac0d7c6178bf6dd469
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/405002
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

sdk/lib/_internal/wasm/lib/js_string_convert.dart

@@ -20,28 +20,14 @@ JSStringImpl? decodeUtf8JS(
   bool allowMalformed,
 ) {
   final length = end - start;
-  if (length >= _shortInputThreshold) {
-    final JSAny? decoder = allowMalformed ? _decoderNonFatal : _decoder;
-    if (decoder != null) {
-      final arrayRef = codeUnits.toJSArrayExternRef(start, length);
-      final textDecoderResult = _useTextDecoder(
-        externRefForJSAny(decoder),
-        arrayRef,
-      );
-      if (textDecoderResult != null) {
-        return textDecoderResult;
-      }
-    }
+  final JSAny? decoder = allowMalformed ? _decoderNonFatal : _decoder;
+  if (decoder != null) {
+    final arrayRef = codeUnits.toJSArrayExternRef(start, length);
+    return _useTextDecoder(externRefForJSAny(decoder), arrayRef);
   }
   return null;
 }
 
-// Always fall back to the Dart implementation for strings shorter than this
-// threshold, as there is a large, constant overhead for using `TextDecoder`.
-// TODO(omersa): This is copied from dart2js runtime, make sure the value is
-// right for dart2wasm.
-const int _shortInputThreshold = 15;
-
 JSStringImpl? _useTextDecoder(
   WasmExternRef? decoder,
   WasmExternRef? codeUnits,