Add Instance::HashCode that matches hashCode.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 37709)
+++ runtime/vm/object.cc	(working copy)
@@ -12858,7 +12858,13 @@
 }
 
 
+RawObject* Instance::HashCode() const {
+  // TODO(koda): Optimize for all builtin classes and all classes
+  // that do not override hashCode.
+  return DartLibraryCalls::HashCode(*this);
+}
 
+
 bool Instance::CanonicalizeEquals(const Instance& other) const {
   if (this->raw() == other.raw()) {
     return true;  // "===".