chore: karma with JS, Dart

Note: karma with dart is still not working
because of how `karma-dart` loads `package:…` dependencies.

Usage:
```
karma start karma-js.conf.js
karma start karma-dart.conf.js
```

Make sure to set `DARTIUM_BIN` env variable.

Refactors `js2dart`:
- live outside of the traceur module (`tools/js2dart/index.js`)
  so it can be reused by gulp and karma
- automatically build the sources in memory,
  so that `js2dart` can be used without running `gulp build` first
- provide a way to specify the moduleName of a compilation run
  independently of the input filename. This helps error messages
  and source maps (not yet enabled) to report the correct file name

Changes project setup:
- add module `test_lib` that contains the primitives for tests
  (e.g. `describe`, `it`, …)
- clean up some sources that had errors in them
- module names in transpiled js and dart files don’t contain
  `lib`, `test` nor `src` any more (e.g. `di/di`).

modules/test_lib/src/test_lib.es6

@@ -0,0 +1,8 @@
+export var describe = window.describe;
+export var it = window.it;
+export var beforeEach = window.beforeEach;
+export var afterEach = window.afterEach;
+export var expect = window.expect;
+
+// To make testing consistent between dart and js
+window.print = window.dump || window.console.log;