Run dartfmt on kernel package

pkg/kernel/testcases/input/optional.dart

 class Foo {
   method(x, [y, z]) {
     return "string";
   }
 }
 
 abstract class External {
   String externalMethod(int x, [int y, int z]);
   void listen(Listener listener);
 }
+
 external External createExternal();
 
 abstract class Listener {
   void event(String input, [int x, int y]);
 }
 
 class TestListener extends Listener {
   void event(input, [x, y]) {}
 }
 
@@ skipping 18 matching lines @@
 
   extern.listen(new TestListener());
   extern.listen(new ExtendedListener());
   extern.listen(new InvalidListener());
 
   var nothing1 = foo.method();
   var nothing2 = foo.method(1, 2, 3, 4);
   var nothing3 = extern.externalMethod();
   var nothing4 = extern.externalMethod(1, 2, 3, 4);
 }