Added heap capacity and used space status methods, for use by status tools.

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
===================================================================
--- runtime/vm/heap.cc	(revision 17153)
+++ runtime/vm/heap.cc	(working copy)
@@ -282,6 +282,16 @@
 }
 
 
+intptr_t Heap::Used(Space space) const {
+  return space == kNew ? new_space_->in_use() : old_space_->in_use();
+}
+
+
+intptr_t Heap::Capacity(Space space) const {
+  return space == kNew ? new_space_->capacity() : old_space_->capacity();
+}
+
+
 void Heap::Profile(Dart_FileWriteCallback callback, void* stream) const {
   HeapProfiler profiler(callback, stream);