Make Error.prototype.toString spec compliant; and fix various side-effect-free error printing metho…

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index d30b2043d239202298d3e627e845bb2e3eee09bc..394dc3af2042ae328546ac0cad675e1981be4a97 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1171,6 +1171,9 @@ class Object {
   MUST_USE_RESULT static MaybeHandle<Object> GetMethod(
       Handle<JSReceiver> receiver, Handle<Name> name);
 
+  // Check whether |object| is an instance of Error or NativeError.
+  static bool IsErrorObject(Isolate* isolate, Handle<Object> object);
+
   // ES6 section 12.5.6 The typeof Operator
   static Handle<String> TypeOf(Isolate* isolate, Handle<Object> object);