Allow some keywords to be treated as identifier names; update ecmaVersion in benchmark.

test/bench.html

@@ -36,7 +36,7 @@ numbers.</p>
 
 <script>
   function runAcorn(code, locations) {
-    acorn.parse(code, {locations: locations});
+    acorn.parse(code, {ecmaVersion: 6, locations: locations});
   }
   function runEsprima(code, locations) {
     esprima.parse(code, {loc: locations});