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

runtime/vm/intermediate_language.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_INTERMEDIATE_LANGUAGE_H_
 #define VM_INTERMEDIATE_LANGUAGE_H_
 
 #include "vm/allocation.h"
 #include "vm/ast.h"
 #include "vm/growable_array.h"
@@ skipping 274 matching lines @@
 
   explicit Value(Definition* definition)
       : definition_(definition),
         previous_use_(NULL),
         next_use_(NULL),
         instruction_(NULL),
         use_index_(-1),
         reaching_type_(NULL) { }
 
   Definition* definition() const { return definition_; }
-  void set_definition(Definition* definition) { definition_ = definition; }
+  void set_definition(Definition* definition) {
+    if ((definition_ != NULL) && (reaching_type_ != NULL)) {
+      UpdateReachingType(definition);
+    }
+    definition_ = definition;
+  }
 
   Value* previous_use() const { return previous_use_; }
   void set_previous_use(Value* previous) { previous_use_ = previous; }
 
   Value* next_use() const { return next_use_; }
   void set_next_use(Value* next) { next_use_ = next; }
 
   bool IsSingleUse() const {
     return (next_use_ == NULL) && (previous_use_ == NULL);
   }
@@ skipping 16 matching lines @@
   // This function must only be used when the new Value is dominated by
   // the original Value.
   Value* CopyWithType() {
     Value* copy = new Value(definition_);
     copy->reaching_type_ = reaching_type_;
     return copy;
   }
 
   CompileType* Type();
 
+  void UpdateReachingType(Definition* new_definition);
+
   void SetReachingType(CompileType* type) {
     reaching_type_ = type;
   }
 
   void PrintTo(BufferFormatter* f) const;
 
   const char* DebugName() const { return "Value"; }
 
   bool IsSmiValue() { return Type()->ToCid() == kSmiCid; }
 
@@ skipping 7724 matching lines @@
   LocationSummary* Name::MakeLocationSummary(Zone* zone, bool opt) const {    \
     UNIMPLEMENTED();                                                          \
     return NULL;                                                              \
   }                                                                           \
   void Name::EmitNativeCode(FlowGraphCompiler* compiler) { UNIMPLEMENTED(); }
 
 
 }  // namespace dart
 
 #endif  // VM_INTERMEDIATE_LANGUAGE_H_