es6-mocha.js

describe('Storage', function() {
  var Storage;

  before(function() {
    return System.import('./storage').then(function(module) {
      Storage = module;
    });
  });

  it('is an object', function() {
    assert.equal(typeof Storage, 'object');
  });
});

 describe('Storage', function() {
   var Storage;

   before(function() {
-    return System.import('./storage');
+    return System.import('./storage').then(function() {
+      Storage = System.get('./storage');
+    });
   });
- 
-  before(function() {
-    Storage = System.get('./storage');
-  });

   it('is an object', function() {
     assert.equal(typeof Storage, 'object');
   });
 });

The only change I made was removing the get call as well since the resolved value of System.import is the module.

Even better!