[flutter_migrate] Implement lineTerminator in the MemoryStdout fake (flutter#6152)

https://dart-review.googlesource.com/c/sdk/+/326761/24/sdk/lib/io/stdio.dart#380 added a `lineTerminator` field to `Stdout`.

See flutter/flutter#143614

Author: jason-simmons

packages/flutter_migrate/test/src/fakes.dart

@@ -132,6 +132,15 @@ class MemoryStdout extends MemoryIOSink implements io.Stdout {
 
   bool _hasTerminal = true;
 
+  @override
+  // ignore: override_on_non_overriding_member
+  String get lineTerminator => '\n';
+  @override
+  // ignore: override_on_non_overriding_member
+  set lineTerminator(String value) {
+    throw UnimplementedError('Setting the line terminator is not supported');
+  }
+
   @override
   io.IOSink get nonBlocking => this;