Allow duplicate property names in classes

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 6578e8dff41044f88133083db03ea4c6ea8c5b91..513ffdc2a9b63e3b49a414886055f35d236322c3 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -176,7 +176,8 @@ var kMessages = {
   module_export_undefined:       ["Export '", "%0", "' is not defined in module"],
   unexpected_super:              ["'super' keyword unexpected here"],
   extends_value_not_a_function:  ["Class extends value ", "%0", " is not a function or null"],
-  prototype_parent_not_an_object: ["Class extends value does not have valid prototype property ", "%0"]
+  prototype_parent_not_an_object: ["Class extends value does not have valid prototype property ", "%0"],
+  duplicate_constructor:         ["A class may only have one constructor"]
 };