Changes to improve the performance of reading a snapshot.

runtime/vm/pages.h

Index: runtime/vm/pages.h
===================================================================
--- runtime/vm/pages.h	(revision 37116)
+++ runtime/vm/pages.h	(working copy)
@@ -125,7 +125,7 @@
   // Returns whether growing to 'after' should trigger a GC.
   // This method can be called before allocation (e.g., pretenuring) or after
   // (e.g., promotion), as it does not change the state of the controller.
-  bool NeedsGarbageCollection(SpaceUsage after) const;
+  bool NeedsGarbageCollection(intptr_t after_capacity_in_words) const;
 
   // Should be called after each collection to update the controller state.
   void EvaluateGarbageCollection(SpaceUsage before,
@@ -202,8 +202,9 @@
                     GrowthPolicy growth_policy = kControlGrowth);
 
   bool NeedsGarbageCollection() const {
-    return page_space_controller_.NeedsGarbageCollection(usage_) ||
-           NeedsExternalGC();
+    const bool needs_gc =
+        page_space_controller_.NeedsGarbageCollection(usage_.capacity_in_words);
+    return  needs_gc || NeedsExternalGC();
   }
 
   intptr_t UsedInWords() const { return usage_.used_in_words; }