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`).

test-main.js

@@ -0,0 +1,12 @@
+var TEST_REGEXP = /^\/base\/modules\/[^\/]*\/test\/.*/;
+
+Object.keys(window.__karma__.files).forEach(function(path) {
+  if (TEST_REGEXP.test(path)) {
+    var moduleName = path
+      .replace(/.*\/modules\//, '')
+      .replace(/\/src\//, '/')
+      .replace(/\/test\//, '/')
+      .replace(/\.\w*$/, '');
+    System.get(moduleName).main();
+  }
+});