Revert r34736 due to slowdown.

runtime/vm/spaces.h

Index: runtime/vm/spaces.h
===================================================================
--- runtime/vm/spaces.h	(revision 34740)
+++ runtime/vm/spaces.h	(working copy)
@@ -1,26 +0,0 @@
-// Copyright (c) 2014, 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.
-
-#ifndef VM_SPACES_H_
-#define VM_SPACES_H_
-
-// This file contains utilities shared by old and new space.
-// TODO(koda): Create Space base class with Space::CurrentUsage().
-
-namespace dart {
-
-// Usage statistics for a space/generation at a particular moment in time.
-struct SpaceUsage {
-  SpaceUsage()
-    : capacity_in_words(0),
-      used_in_words(0),
-      external_in_words(0) {}
-  intptr_t capacity_in_words;
-  intptr_t used_in_words;
-  intptr_t external_in_words;
-};
-
-}  // namespace dart
-
-#endif  // VM_SPACES_H_