first cut of async/await (#2098)

- async arrow functions not yet supported


fixes #1789

lib/scope.js

@@ -590,6 +590,8 @@ AST_Toplevel.DEFMETHOD("compute_char_frequency", function(options){
             base54.consider("finally");
         else if (node instanceof AST_Yield)
             base54.consider("yield");
+        else if (node instanceof AST_Await)
+            base54.consider("await");
         else if (node instanceof AST_Symbol && node.unmangleable(options))
             base54.consider(node.name);
         else if (node instanceof AST_Unary || node instanceof AST_Binary)