Use dartfmt on Observatory code

runtime/observatory/lib/src/repositories/retained_size.dart

Index: runtime/observatory/lib/src/repositories/retained_size.dart
diff --git a/runtime/observatory/lib/src/repositories/retained_size.dart b/runtime/observatory/lib/src/repositories/retained_size.dart
index 3869254bf505c088de5d2b12e7d82d35050b393f..6d3cef52a0db54d83f9f46733d852f268286d957 100644
--- a/runtime/observatory/lib/src/repositories/retained_size.dart
+++ b/runtime/observatory/lib/src/repositories/retained_size.dart
@@ -8,9 +8,8 @@ class RetainedSizeRepository implements M.RetainedSizeRepository {
   Future<M.Guarded<M.Instance>> get(M.IsolateRef i, String id) async {
     S.Isolate isolate = i as S.Isolate;
     assert(isolate != null);
-    final response = await isolate.invokeRpc('_getRetainedSize', {
-        'targetId' : id
-    });
+    final response =
+        await isolate.invokeRpc('_getRetainedSize', {'targetId': id});
     return new S.Guarded<S.Instance>(response);
   }
 }