UnboxDouble: convert smi constants to doubles at compile time instead of at runtime.

runtime/vm/flow_graph_optimizer.cc

Index: runtime/vm/flow_graph_optimizer.cc
===================================================================
--- runtime/vm/flow_graph_optimizer.cc	(revision 16291)
+++ runtime/vm/flow_graph_optimizer.cc	(working copy)
@@ -192,7 +192,17 @@
     const intptr_t deopt_id = (deopt_target != NULL) ?
         deopt_target->DeoptimizationTarget() : Isolate::kNoDeoptId;
     ASSERT((deopt_target != NULL) || (def->GetPropagatedCid() == kDoubleCid));
-    converted = new UnboxDoubleInstr(new Value(def), deopt_id);
+    if (def->IsConstant() && def->AsConstant()->value().IsSmi()) {
+      const double dbl_val =
+          Smi::Cast(def->AsConstant()->value()).AsDoubleValue();
+      const Double& dbl_obj =
+          Double::ZoneHandle(Double::New(dbl_val, Heap::kOld));
+      ConstantInstr* double_const = new ConstantInstr(dbl_obj);
+      InsertBefore(instr, double_const, NULL, Definition::kValue);
+      converted = new UnboxDoubleInstr(new Value(double_const), deopt_id);
+    } else {
+      converted = new UnboxDoubleInstr(new Value(def), deopt_id);
+    }
   }
   ASSERT(converted != NULL);
   InsertBefore(instr, converted, use->instruction()->env(),