Emit a warning when a superclass method is not called

``` js
// ==ClosureCompiler==
// @compilation_level ADVANCED_OPTIMIZATIONS
// @output_file_name default.js
// @use_closure_library true
// @warning_level VERBOSE
// ==/ClosureCompiler==

goog.require('goog.events.EventTarget');

/**
* @constructor
* @extends {goog.events.EventTarget}
*/
var f = function() {
};

/** @override */
f.prototype.dispatchEvent = function(e) {};
```

Since dispatchEvent doesn't call goog.base(this, 'dispatchEvent') or equivalent, we should emit an optional warning but we currently do not. Similar to https://github.com/google/closure-compiler/issues/278 which is about constructors not methods.