Allow this.state.clone() when parsing decorators (#262)

Adding the line `this.state = this.state.clone();` in a parser plugin 
(specifically, in my case `parseExprAtom`)
would break decorators. 

This change fixes that.

src/parser/statement.js

@@ -149,7 +149,8 @@ pp.takeDecorators = function (node) {
 
 pp.parseDecorators = function (allowExport) {
   while (this.match(tt.at)) {
-    this.state.decorators.push(this.parseDecorator());
+    let decorator = this.parseDecorator();
+    this.state.decorators.push(decorator);
   }
 
   if (allowExport && this.match(tt._export)) {