DownDuration = function (key, duration)
{
    if (duration === undefined) { duration = 50; }

    var current = key.plugin.game.loop.time - key.timeDown;

    return (key.isDown && current < duration);
}
Returns `true` if the Key was pressed down within the `duration` value given, based on the current
game clock time. Or `false` if it either isn't down, or was pressed down longer ago than the given duration.

@function Phaser.Input.Keyboard.DownDuration
@since 3.0.0

@param {Phaser.Input.Keyboard.Key} key - The Key object to test.
@param {number} [duration=50] - The duration, in ms, within which the key must have been pressed down.

@return {boolean} `true` if the Key was pressed down within `duration` ms ago, otherwise `false`.