Add checks to runtime functions.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 720d96e0ea22d891ee22f4668b2cd4a866dc8d91..02d3ec2e97cea4e260cd35d333e4972ddc06cc9d 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -6084,7 +6084,7 @@ class JSFunction: public JSObject {
 
   // After prototype is removed, it will not be created when accessed, and
   // [[Construct]] from this function will not be allowed.
-  Object* RemovePrototype();
+  void RemovePrototype();
   inline bool should_have_prototype();
 
   // Accessor for this function's initial map's [[class]]
@@ -6096,7 +6096,7 @@ class JSFunction: public JSObject {
   // Instances created afterwards will have a map whose [[class]] is
   // set to 'value', but there is no guarantees on instances created
   // before.
-  Object* SetInstanceClassName(String* name);
+  void SetInstanceClassName(String* name);
 
   // Returns if this function has been compiled to native code yet.
   inline bool is_compiled();