Propagate this-specialization to regular (megamorphic) calls

runtime/vm/flow_graph.cc

Index: runtime/vm/flow_graph.cc
diff --git a/runtime/vm/flow_graph.cc b/runtime/vm/flow_graph.cc
index 92c75ac3f2f97f5e7a6754d2b35dc5ee02d4fb3d..8a3a69155d0fb6b5ac6c536198d5b206f2ff4d95 100644
--- a/runtime/vm/flow_graph.cc
+++ b/runtime/vm/flow_graph.cc
@@ -83,11 +83,15 @@ void FlowGraph::ReplaceCurrentInstruction(ForwardInstructionIterator* iterator,
     }
   }
   if (current->ArgumentCount() != 0) {
-    // This is a call instruction. Must remove original push arguments.
+    // Replacing a call instruction with something else.  Must remove
+    // superfluous push arguments.
     for (intptr_t i = 0; i < current->ArgumentCount(); ++i) {
       PushArgumentInstr* push = current->PushArgumentAt(i);
-      push->ReplaceUsesWith(push->value()->definition());
-      push->RemoveFromGraph();
+      if (replacement == NULL || i >= replacement->ArgumentCount() ||
+          replacement->PushArgumentAt(i) != push) {
+        push->ReplaceUsesWith(push->value()->definition());
+        push->RemoveFromGraph();
+      }
     }
   }
   iterator->RemoveCurrentFromGraph();