Formatter workaround for missing abstract keyword in ClassTypeAliases (dartbug.com/15912).

pkg/analyzer/test/services/data/cu_tests.data

 >>>
 class
 A
 {
 }
 <<<
 class A {
 }
 >>> dartbug.com/15876
 // fisk
 import "dart:typed_data";
 
 main() {
   ByteData byteData = new ByteData(1);
   print(byteData is ByteData);
   print(byteData.elementSizeInBytes);
 }
 <<<
 // fisk
 import "dart:typed_data";
 
 main() {
   ByteData byteData = new ByteData(1);
   print(byteData is ByteData);
   print(byteData.elementSizeInBytes);
-}
+}
+>>>
+/**
+ * Y.
+ */
+abstract class Y = String;
+<<<
+/**
+ * Y.
+ */
+abstract class Y = String;