Add web serial support and USB tests

lib/services/usb_serial_frame_codec.dart

@@ -0,0 +1,70 @@
+import 'dart:typed_data';
+
+const int usbSerialTxFrameStart = 0x3c;
+const int usbSerialRxFrameStart = 0x3e;
+const int usbSerialHeaderLength = 3;
+
+Uint8List wrapUsbSerialTxFrame(Uint8List payload) {
+  final packet = Uint8List(usbSerialHeaderLength + payload.length);
+  packet[0] = usbSerialTxFrameStart;
+  packet[1] = payload.length & 0xff;
+  packet[2] = (payload.length >> 8) & 0xff;
+  packet.setRange(usbSerialHeaderLength, packet.length, payload);
+  return packet;
+}
+
+class UsbSerialDecodedPacket {
+  const UsbSerialDecodedPacket({
+    required this.frameStart,
+    required this.payload,
+  });
+
+  final int frameStart;
+  final Uint8List payload;
+
+  bool get isRxFrame => frameStart == usbSerialRxFrameStart;
+}
+
+class UsbSerialFrameDecoder {
+  final List<int> _rxBuffer = <int>[];
+
+  List<UsbSerialDecodedPacket> ingest(Uint8List bytes) {
+    if (bytes.isEmpty) {
+      return const <UsbSerialDecodedPacket>[];
+    }
+
+    _rxBuffer.addAll(bytes);
+    final packets = <UsbSerialDecodedPacket>[];
+
+    while (true) {
+      if (_rxBuffer.isEmpty) {
+        return packets;
+      }
+
+      if (_rxBuffer.first != usbSerialRxFrameStart &&
+          _rxBuffer.first != usbSerialTxFrameStart) {
+        _rxBuffer.removeAt(0);
+        continue;
+      }
+
+      if (_rxBuffer.length < usbSerialHeaderLength) {
+        return packets;
+      }
+
+      final payloadLength = _rxBuffer[1] | (_rxBuffer[2] << 8);
+      final packetLength = usbSerialHeaderLength + payloadLength;
+      if (_rxBuffer.length < packetLength) {
+        return packets;
+      }
+
+      final frameStart = _rxBuffer.first;
+      final payload = Uint8List.fromList(
+        _rxBuffer.sublist(usbSerialHeaderLength, packetLength),
+      );
+      _rxBuffer.removeRange(0, packetLength);
+      packets.add(
+        UsbSerialDecodedPacket(frameStart: frameStart, payload: payload),
+      );
+    }
+  }
+}