Use dartfmt on Observatory code

runtime/observatory/lib/src/models/objects/class.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of models;
 
 abstract class ClassRef extends ObjectRef {
   /// The name of this class.
   String get name;
 }
@@ skipping 54 matching lines @@
   Allocations get oldSpace;
 
   bool get traceAllocations;
 }
 
 abstract class InstanceSet {
   int get count;
   Iterable<ObjectRef> get samples;
 }
 
-abstract class TopRetainedInstances {
-}
+abstract class TopRetainedInstances {}