Cache hash code for closures.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 8cf284d8107bfafacc8001314c8d91536a14720c..b56b3dfed266e92983e4229d6630bd9fda2a66bb 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -862,6 +862,7 @@ class RawClosureData : public RawObject {
   RawFunction* parent_function_;  // Enclosing function of this local function.
   RawType* signature_type_;
   RawInstance* closure_;  // Closure object for static implicit closures.
+  RawObject* hash_;
   RawObject** to() { return reinterpret_cast<RawObject**>(&ptr()->closure_); }
 
   friend class Function;