code: function() {
      let result = null;
      if (this.token === this.tok.T_AS) {
        if (this.next().expect(this.tok.T_STRING)) {
          const aliasName = this.node("identifier");
          const name = this.text();
          this.next();
          result = aliasName(name);
        }
      }
      return result;
    }
docs: Reads a use statement
  ```ebnf
  use_alias ::= (T_AS T_STRING)?
  ```
  @return {String|null}