src/shared/index.js/dispatchObservers
export function dispatchObservers ( component, group, newState, oldState ) {
	for ( var key in group ) {
		if ( !( key in newState ) ) continue;

		var newValue = newState[ key ];
		var oldValue = oldState[ key ];

		if ( differs( newValue, oldValue ) ) {
			var callbacks = group[ key ];
			if ( !callbacks ) continue;

			for ( var i = 0; i < callbacks.length; i += 1 ) {
				var callback = callbacks[i];
				if ( callback.__calling ) continue;

				callback.__calling = true;
				callback.call( component, newValue, oldValue );
				callback.__calling = false;
			}
		}
	}
}