eatIdName_() {
    let t = nextToken();
    if (t.type !== IDENTIFIER) {
      if (!t.isKeyword()) {
        this.reportExpectedError_(t, 'identifier');
        return null;
      }
      return new IdentifierToken(t.location, t.type);
    }
    return t;
  }
Eats an identifier or keyword. Equivalent to IdentifierName in the spec.

@return {Token}
@private