[dart:io] Fix ProcessStartMode.values

Change-Id: I1bdd98c7e86991e38d6bc7555620d784d595eb2d
Reviewed-on: https://dart-review.googlesource.com/54441
Reviewed-by: Leaf Petersen <leafp@google.com>
Commit-Queue: Zach Anderson <zra@google.com>
diff --git a/sdk/lib/io/process.dart b/sdk/lib/io/process.dart
index e1d4c23..3c7e1c2 100644
--- a/sdk/lib/io/process.dart
+++ b/sdk/lib/io/process.dart
@@ -159,7 +159,7 @@
   @Deprecated("Use detachedWithStdio instead")
   static const DETACHED_WITH_STDIO = detachedWithStdio;
 
-  List<ProcessStartMode> get values => const <ProcessStartMode>[
+  static List<ProcessStartMode> get values => const <ProcessStartMode>[
         normal,
         inheritStdio,
         detached,