Adding support for CSP-compliant js sources.

Adding support for packaged apps (e.g. Chrome Apps, Firefox OS _privileged_ [1] apps) and general CSP-enabled  web apps.

To enable this feature -- which rewrites Dart source imports to `source.precompiled.js` -- just add this to _pubspec.yaml_:

```
transformers:
- dart_to_js_script_rewriter:
    csp: true
```

[1] https://developer.mozilla.org/en-US/Marketplace/Publishing/Packaged_apps#Types_of_packaged_apps