A reference to the topmost window in the window hierarchy.
If this [WindowBase] is the topmost [WindowBase], [top] will return a reference to the [WindowBase] itself.
// Add an IFrame to the current window.
IFrameElement myIFrame = new IFrameElement();
window.document.body.elements.add(myIFrame);
// Add an IFrame inside of the other IFrame.
IFrameElement innerIFrame = new IFrameElement();
myIFrame.elements.add(innerIFrame);
print(myIframe.contentWindow.top == window) // 'true'
print(innerIFrame.contentWindow.top == window) // 'true'
print(window.top == window) // 'true'