I have a lightning component on the app exchange that provides a canvas element for the user to draw their signature. In a Summer 16 pre-release org with the lightning locker service enabled, when I attempt to get the drawing context via :

this.ctx = this.canvas.getContext('2d');

I get the following error:

    TypeError: this.canvas.getContext is not a function. (In 
      'this.canvas.getContext("2d")', 'this.canvas.getContext' is undefined)