1. DomName('WebGL2RenderingContext.viewport')
  2. DocsEditable()
  3. Experimental()
void viewport(int x, int y, int width, int height)

Source 

@DomName('WebGL2RenderingContext.viewport')
@DocsEditable()
@Experimental() // untriaged
void viewport(int x, int y, int width, int height) => _blink.BlinkWebGL2RenderingContext.instance.viewport_Callback_4_(this, x, y, width, height);