Reapply "Improve hashCode for closure objects" with fixes.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 62d6344021e9fd5afaf20afe5971f6a315fa9083..82a88e6cff619ccf8043077079e33bfbd779663f 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -2410,7 +2410,7 @@ class Function : public Object {
 
   RawInstance* ImplicitInstanceClosure(const Instance& receiver) const;
 
-  RawSmi* GetClosureHashCode() const;
+  intptr_t ComputeClosureHash() const;
 
   // Redirection information for a redirecting factory.
   bool IsRedirectingFactory() const;
@@ -3081,9 +3081,6 @@ class ClosureData : public Object {
   RawInstance* implicit_static_closure() const { return raw_ptr()->closure_; }
   void set_implicit_static_closure(const Instance& closure) const;
 
-  RawObject* hash() const { return raw_ptr()->hash_; }
-  void set_hash(intptr_t value) const;
-
   static RawClosureData* New();
 
   FINAL_HEAP_OBJECT_IMPLEMENTATION(ClosureData, Object);
@@ -5541,6 +5538,9 @@ class Instance : public Object {
   // Equivalent to invoking hashCode on this instance.
   virtual RawObject* HashCode() const;
 
+  // Equivalent to invoking identityHashCode with this instance.
+  RawObject* IdentityHashCode() const;
+
   static intptr_t InstanceSize() {
     return RoundedAllocationSize(sizeof(RawInstance));
   }
@@ -8356,6 +8356,9 @@ class Closure : public Instance {
   RawContext* context() const { return raw_ptr()->context_; }
   static intptr_t context_offset() { return OFFSET_OF(RawClosure, context_); }
 
+  RawSmi* hash() const { return raw_ptr()->hash_; }
+  static intptr_t hash_offset() { return OFFSET_OF(RawClosure, hash_); }
+
   static intptr_t InstanceSize() {
     return RoundedAllocationSize(sizeof(RawClosure));
   }
@@ -8367,6 +8370,8 @@ class Closure : public Instance {
     return true;
   }
 
+  int64_t ComputeHash() const;
+
   static RawClosure* New(const TypeArguments& instantiator_type_arguments,
                          const TypeArguments& function_type_arguments,
                          const Function& function,