Clarify e2e readme for running on firebase test lab (flutter#2898)

* clarify e2e on firebase

Author: CareF

packages/e2e/README.md

@@ -130,10 +130,12 @@ documentation](https://firebase.google.com/docs/test-lab/?gclid=EAIaIQobChMIs5qV
 to set up a project.
 
 To run an e2e test on Android devices using Firebase Test Lab, use gradle commands to build an
-instrumentation test for Android.
+instrumentation test for Android, after creating `androidTest` as suggested in the last section.
 
-```
+```bash
 pushd android
+# flutter build generates files in android/ for building the app
+flutter build apk
 ./gradlew app:assembleAndroidTest
 ./gradlew app:assembleDebug -Ptarget=<path_to_test>.dart
 popd
@@ -142,7 +144,7 @@ popd
 Upload the build apks Firebase Test Lab, making sure to replace <PATH_TO_KEY_FILE>,
 <PROJECT_NAME>, <RESULTS_BUCKET>, and <RESULTS_DIRECTORY> with your values.
 
-```
+```bash
 gcloud auth activate-service-account --key-file=<PATH_TO_KEY_FILE>
 gcloud --quiet config set project <PROJECT_NAME>
 gcloud firebase test android run --type instrumentation \