enable lint prefer_equal_for_default_values (#18156)

diff --git a/packages/flutter_tools/lib/src/base/utils.dart b/packages/flutter_tools/lib/src/base/utils.dart
index 735ab94..aa1eaa9 100644
--- a/packages/flutter_tools/lib/src/base/utils.dart
+++ b/packages/flutter_tools/lib/src/base/utils.dart
@@ -239,7 +239,7 @@
 ///   - has a different initial value for the first callback delay
 ///   - waits for a callback to be complete before it starts the next timer
 class Poller {
-  Poller(this.callback, this.pollingInterval, { this.initialDelay: Duration.zero }) {
+  Poller(this.callback, this.pollingInterval, { this.initialDelay = Duration.zero }) {
     new Future<Null>.delayed(initialDelay, _handleCallback);
   }