Add Instance::HashCode that matches hashCode.

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 37709)
+++ runtime/vm/object.h	(working copy)
@@ -4248,6 +4248,9 @@
                       const Array& param_names,
                       const Array& param_values) const;
 
+  // Equivalent to invoking hashCode on this instance.
+  virtual RawObject* HashCode() const;
+
   static intptr_t InstanceSize() {
     return RoundedAllocationSize(sizeof(RawInstance));
   }
@@ -4946,6 +4949,8 @@
     return false;
   }
 
+  virtual RawObject* HashCode() const { return raw(); }
+
   // Integer is an abstract class.
   virtual bool IsZero() const {
     UNREACHABLE();
@@ -5317,6 +5322,8 @@
   static intptr_t Hash(const uint16_t* characters, intptr_t len);
   static intptr_t Hash(const int32_t* characters, intptr_t len);
 
+  virtual RawObject* HashCode() const { return Integer::New(Hash()); }
+
   int32_t CharAt(intptr_t index) const;
 
   Scanner::CharAtFunc CharAtFunc() const;