Implement parsing of ES6 Rest Parameters

src/scopes.h

Index: src/scopes.h
diff --git a/src/scopes.h b/src/scopes.h
index 56a7ee97657148e430de38d8344f37cb5d2ec4cf..2b9ece2ad0bfabc6b0272242852d48b256ef4ea5 100644
--- a/src/scopes.h
+++ b/src/scopes.h
@@ -467,6 +467,14 @@ class Scope: public ZoneObject {
     return variables_.Lookup(name) != NULL;
   }
 
+  bool IsDeclaredParameter(const AstRawString* name) {
+    // If IsSimpleParameterList is false, duplicate parameters are not allowed,
+    // however `arguments` may be allowed if function is not strict code. Thus,
+    // the assumptions explained above do not hold.
+    Variable* var = variables_.Lookup(name);
+    return var != NULL && params_.Contains(var);
+  }
+
   // ---------------------------------------------------------------------------
   // Debugging.