Don't constant fold operation if result is incompatible with it.

runtime/vm/flow_graph_type_propagator.cc

Index: runtime/vm/flow_graph_type_propagator.cc
diff --git a/runtime/vm/flow_graph_type_propagator.cc b/runtime/vm/flow_graph_type_propagator.cc
index bc052f687d02d76610e55807d24aed3c1df0cd42..74f28a6512b78b8211d492b01bb43e22cb36aa65 100644
--- a/runtime/vm/flow_graph_type_propagator.cc
+++ b/runtime/vm/flow_graph_type_propagator.cc
@@ -661,6 +661,13 @@ bool CompileType::IsMoreSpecificThan(const AbstractType& other) {
 }
 
 
+void Value::UpdateReachingType(Definition* new_definition) {
+  if (definition()->Type() == reaching_type_) {
+    reaching_type_ = NULL;
+  }
+}
+
+
 CompileType* Value::Type() {
   if (reaching_type_ == NULL) {
     reaching_type_ = definition()->Type();