made ascii string encoding faster (#101777)

dev/benchmarks/microbenchmarks/lib/foundation/standard_message_codec_bench.dart

@@ -92,5 +92,20 @@ void main() {
 
   watch.reset();
 
+  watch.start();
+  for (int i = 0; i < _kNumIterations; i += 1) {
+    codec.encodeMessage('special chars >\u263A\u{1F602}<');
+  }
+  watch.stop();
+
+  printer.addResult(
+    description: 'StandardMessageCodec unicode',
+    value: watch.elapsedMicroseconds.toDouble() / _kNumIterations,
+    unit: 'us per iteration',
+    name: 'StandardMessageCodec_unicode',
+  );
+
+  watch.reset();
+
   printer.printToStdout();
 }

packages/flutter/lib/src/services/message_codecs.dart

@@ -386,9 +386,28 @@ class StandardMessageCodec implements MessageCodec<Object?> {
       }
     } else if (value is String) {
       buffer.putUint8(_valueString);
-      final Uint8List bytes = utf8.encoder.convert(value);
-      writeSize(buffer, bytes.length);
-      buffer.putUint8List(bytes);
+      final Uint8List asciiBytes = Uint8List(value.length);
+      Uint8List? utf8Bytes;
+      int utf8Offset = 0;
+      // Only do utf8 encoding if we encounter non-ascii characters.
+      for (int i = 0; i < value.length; i += 1) {
+        final int char = value.codeUnitAt(i);
+        if (char <= 0x7f) {
+          asciiBytes[i] = char;
+        } else {
+          utf8Bytes = utf8.encoder.convert(value.substring(i));
+          utf8Offset = i;
+          break;
+        }
+      }
+      if (utf8Bytes != null) {
+        writeSize(buffer, utf8Offset + utf8Bytes.length);
+        buffer.putUint8List(Uint8List.sublistView(asciiBytes, 0, utf8Offset));
+        buffer.putUint8List(utf8Bytes);
+      } else {
+        writeSize(buffer, asciiBytes.length);
+        buffer.putUint8List(asciiBytes);
+      }
     } else if (value is Uint8List) {
       buffer.putUint8(_valueUint8List);
       writeSize(buffer, value.length);