addonPreprocessTree(type, tree) {
    return addonProcessTree(this.project, 'preprocessTree', type, tree);
  }
Runs addon pre-processing on a given tree and returns the processed tree.

    This enables addons to do process immediately **before** the preprocessor for a
    given type is run.  If an addon wishes to apply a transform  after the
    preprocessors run, they can instead implement the postprocessTree hook.

    To utilize this addons implement `preprocessTree` hook.

    An example, would be to remove some set of files before the preprocessors run.

    ```js
    var stew = require('broccoli-stew');

    module.exports = {
      name: 'my-cool-addon',
      preprocessTree(type, tree) {
        if (type === 'js' && type === 'template') {
          return stew.rm(tree, someGlobPattern);
        }

        return tree;
      }
    }
    ```

    @private
    @method addonPreprocessTree
    @param  {String} type Type of tree
    @param  {Tree}   tree Tree to process
    @return {Tree}        Processed tree