Fixes bug in calculation of memory used by new space

runtime/vm/scavenger.h

 // Copyright (c) 2012, 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 RUNTIME_VM_SCAVENGER_H_
 #define RUNTIME_VM_SCAVENGER_H_
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/dart.h"
@@ skipping 183 matching lines @@
 
   uword top() { return top_; }
   uword end() { return end_; }
 
   void set_top(uword value) { top_ = value; }
   void set_end(uword value) {
     ASSERT(to_->end() == value);
     end_ = value;
   }
 
-  int64_t UsedInWords() const {
-    return (top_ - FirstObjectStart()) >> kWordSizeLog2;
-  }
+  int64_t UsedInWords() const;
   int64_t CapacityInWords() const { return to_->size_in_words(); }
   int64_t ExternalInWords() const { return external_size_ >> kWordSizeLog2; }
   SpaceUsage GetCurrentUsage() const {
     SpaceUsage usage;
     usage.used_in_words = UsedInWords();
     usage.capacity_in_words = CapacityInWords();
     usage.external_in_words = ExternalInWords();
     return usage;
   }
 
@@ skipping 121 matching lines @@
 
   friend class ScavengerVisitor;
   friend class ScavengerWeakVisitor;
 
   DISALLOW_COPY_AND_ASSIGN(Scavenger);
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_SCAVENGER_H_