Fail with fatal error instead of segmentation fault if virtual memory allocation fails.

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
===================================================================
--- runtime/vm/scavenger.cc	(revision 19923)
+++ runtime/vm/scavenger.cc	(working copy)
@@ -319,7 +319,9 @@
 
   // Allocate the virtual memory for this scavenge heap.
   space_ = VirtualMemory::Reserve(max_capacity);
-  ASSERT(space_ != NULL);
+  if (space_ == NULL) {
+    FATAL("Out of memory.\n");
+  }
 
   // Allocate the entire space at the beginning.
   space_->Commit(false);