Add @override tag support

[demxa]

It used at Google JS Style Guide:

@override For example:

/**
 * @return {string} Human-readable representation of project.SubClass.
 * @override
 */
project.SubClass.prototype.toString = function() {
  // ...
};

https://google.github.io/styleguide/javascriptguide.xml?showone=Comments#Comments

[ricordisamoa]

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