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

Source 

@DomName('WebGLRenderingContext.viewport')
@DocsEditable()
void viewport(int x, int y, int width, int height) => _blink.BlinkWebGLRenderingContext.instance.viewport_Callback_4_(unwrap_jso(this), x, y, width, height);