Reduce GC from external allocation.

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
===================================================================
--- runtime/vm/scavenger.h	(revision 34384)
+++ runtime/vm/scavenger.h	(working copy)
@@ -160,14 +160,12 @@
   // not consume space in the to space they leave enough room for this stack.
   void PushToPromotedStack(uword addr) {
     ASSERT(scavenging_);
-    ASSERT(external_size_ == 0);
     end_ -= sizeof(addr);
     ASSERT(end_ > top_);
     *reinterpret_cast<uword*>(end_) = addr;
   }
   uword PopFromPromotedStack() {
     ASSERT(scavenging_);
-    ASSERT(external_size_ == 0);
     uword result = *reinterpret_cast<uword*>(end_);
     end_ += sizeof(result);
     ASSERT(end_ <= to_->end());
@@ -175,7 +173,6 @@
   }
   bool PromotedStackHasMore() const {
     ASSERT(scavenging_);
-    ASSERT(external_size_ == 0);
     return end_ < to_->end();
   }
 
@@ -211,7 +208,6 @@
   intptr_t collections_;
 
   // The total size of external data associated with objects in this scavenger.
-  // External allocations decrease end_. If promoted stack is in use, this is 0.
   intptr_t external_size_;
 
   friend class ScavengerVisitor;