1. @DomName('WebGL2RenderingContext.beginTransformFeedback')
  2. @DocsEditable()
  3. @Experimental()
void beginTransformFeedback(int primitiveMode)

Source 

@DomName('WebGL2RenderingContext.beginTransformFeedback')
@DocsEditable()
@Experimental() // untriaged
void beginTransformFeedback(int primitiveMode) =>
    _blink.BlinkWebGL2RenderingContext.instance
        .beginTransformFeedback_Callback_1_(this, primitiveMode);