Move object literal checking into checker classes

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 39887d68142cdca9bcaf0ee6c5a64ffc07b37e5d..eeacaf386ae0c3c471eef20a4e0f2dac8b794bd0 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -184,7 +184,8 @@ var kMessages = {
   prototype_parent_not_an_object: ["Class extends value does not have valid prototype property ", "%0"],
   duplicate_constructor:         ["A class may only have one constructor"],
   sloppy_lexical:                ["Block-scoped declarations (let, const, function, class) not yet supported outside strict mode"],
-  super_constructor_call:        ["A 'super' constructor call may only appear as the first statement of a function, and its arguments may not access 'this'. Other forms are not yet supported."]
+  super_constructor_call:        ["A 'super' constructor call may only appear as the first statement of a function, and its arguments may not access 'this'. Other forms are not yet supported."],
+  duplicate_proto:               ["Duplicate __proto__ fields are not allowed in object literals"]
 };