perf: Use for loops while cloning classes

packages/babel-parser/src/tokenizer/state.js

@@ -178,7 +178,9 @@ export default class State {
 
   clone(skipArrays?: boolean): State {
     const state = new State();
-    Object.keys(this).forEach(key => {
+    const keys = Object.keys(this);
+    for (let i = 0, length = keys.length; i < length; i++) {
+      const key = keys[i];
       // $FlowIgnore
       let val = this[key];
 
@@ -188,7 +190,8 @@ export default class State {
 
       // $FlowIgnore
       state[key] = val;
-    });
+    }
+
     return state;
   }
 }