Filter invalid characters in the terminal input stream (#18892)

Fixes https://github.com/flutter/flutter/issues/18007

packages/flutter_tools/lib/src/base/terminal.dart

@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:convert' show ascii;
+import 'dart:convert' show AsciiDecoder;
 
 import 'package:quiver/strings.dart';
 
@@ -87,7 +87,7 @@ class AnsiTerminal {
   ///
   /// Useful when the console is in [singleCharMode].
   Stream<String> get onCharInput {
-    _broadcastStdInString ??= io.stdin.transform(ascii.decoder).asBroadcastStream();
+    _broadcastStdInString ??= io.stdin.transform(const AsciiDecoder(allowInvalid: true)).asBroadcastStream();
     return _broadcastStdInString;
   }