dispose method
void
dispose()
Implementation
void dispose() {
// FIXME: for break circle reference
viewport.controller = null;
debugDOMTreeChanged = null;
_teardownObserver();
_unregisterPlatformBrightnessChange();
// Should clear previous page cached ui commands
clearUICommand(_contextId);
disposePage(_contextId);
_clearTargets();
document.dispose();
window.dispose();
_disposed = true;
}