Ensure errors launching emulators are exposed to the daemon (#18446)

packages/flutter_tools/test/emulator_test.dart

diff --git a/packages/flutter_tools/test/emulator_test.dart b/packages/flutter_tools/test/emulator_test.dart
index 30b411a..be36d5f 100644
--- a/packages/flutter_tools/test/emulator_test.dart
+++ b/packages/flutter_tools/test/emulator_test.dart
@@ -62,7 +62,7 @@
   final String label;
 
   @override
-  Future<bool> launch() {
+  Future<void> launch() {
     throw new UnimplementedError('Not implemented in Mock');
   }
 }