use array to speed up sbc lookup

lib/terminal/sbc.dart

@@ -2,18 +2,23 @@ import 'package:xterm/terminal/terminal.dart';
 
 typedef SbcHandler = void Function(int, Terminal);
 
-final sbcHandlers = <int, SbcHandler>{
-  0x05: _voidHandler,
-  0x07: _bellHandler,
-  0x08: _backspaceReturnHandler,
-  0x09: _tabHandler,
-  0x0a: _newLineHandler,
-  0x0b: _newLineHandler,
-  0x0c: _newLineHandler,
-  0x0d: _carriageReturnHandler,
-  0x0e: _shiftOutHandler,
-  0x0f: _shiftInHandler,
-};
+final sbcHandlers = _sbcHandlers();
+
+// build single byte character lookup table
+List<SbcHandler?> _sbcHandlers() {
+  final result = List<SbcHandler?>.filled(0x0f, null);
+  result[0x05] = _voidHandler;
+  result[0x07] = _bellHandler;
+  result[0x08] = _backspaceReturnHandler;
+  result[0x09] = _tabHandler;
+  result[0x0a] = _newLineHandler;
+  result[0x0b] = _newLineHandler;
+  result[0x0c] = _newLineHandler;
+  result[0x0d] = _carriageReturnHandler;
+  result[0x0e] = _shiftOutHandler;
+  result[0x0f] = _shiftInHandler;
+  return result;
+}
 
 void _bellHandler(int code, Terminal terminal) {
   terminal.onBell();