fix an issue where the refresh benchmark would overwrite a completer (#4315)

packages/flutter_tools/lib/src/observatory.dart

diff --git a/packages/flutter_tools/lib/src/observatory.dart b/packages/flutter_tools/lib/src/observatory.dart
index 5e7a778..7bcc8e1 100644
--- a/packages/flutter_tools/lib/src/observatory.dart
+++ b/packages/flutter_tools/lib/src/observatory.dart
@@ -83,7 +83,7 @@
     if (isolates.isNotEmpty)
       return isolates.first;
 
-    _waitFirstIsolateCompleter = new Completer<IsolateRef>();
+    _waitFirstIsolateCompleter ??= new Completer<IsolateRef>();
 
     getVM().then((VM vm) {
       for (IsolateRef isolate in vm.isolates)