source map support in testing

[rogerz]

It seems currently the test command is locating the error in build.js. Is it possible to make it to the original source code?

  1) grid should sample the canvas to grid:
     TypeError: 'undefined' is not an object (evaluating 'this.options.scale')
      at http://localhost:51934/build.js:5399
      at http://localhost:51934/build.js:119
      at callFn (http://localhost:51934/mocha.js:4452)