isDevMode
返回 Angular 是否处于开发模式。调用一次后，该值将被锁定，并且将不再更改。
Returns whether Angular is in development mode. After called once, the value is locked and won't change any more.
说明
默认情况下，这是正确的，除非用户在调用它之前调用 enableProdMode
By default, this is true, unless a user calls enableProdMode before calling this.
