Android Studio is a new Android development environment based on IntelliJ IDEA, Gradle-based build support. It have different project structure.
In Eclipse, assets is in /assets folder under your project. In Android Studio, it is in /src/main/assets. In both Eclipse and Android Studio, /assets have the same level of /res folder.
Compare with the exercise of "Embed html using Google Maps JavaScript API v3 in Android App", where /assets/simplemap.html is a asset file of HTML. In Android Studio, it in /src/main/assets/simplemap.html. In both case, it can be accessed with "file:///android_asset/simplemap.html".
reference: Gradle Plugin User Guide > Project Structure