Add flutter_assets and *.class to gitignore (#13664)

.gitignore

 # Miscellaneous
+*.class
 *.lock
 *.log
 *.pyc
@@ -69,6 +70,7 @@ unlinked_spec.ds
 **/ios/Flutter/Generated.xcconfig
 **/ios/Flutter/app.flx
 **/ios/Flutter/app.zip
+**/ios/Flutter/flutter_assets/
 **/ios/ServiceDefinitions.json
 
 # Exceptions to above rules.

packages/flutter_tools/templates/create/android.tmpl/.gitignore

 *.iml
+*.class
 .gradle
 /local.properties
 /.idea/workspace.xml

packages/flutter_tools/templates/create/ios.tmpl/.gitignore

@@ -33,6 +33,7 @@ Icon?
 
 /Flutter/app.flx
 /Flutter/app.zip
+/Flutter/flutter_assets/
 /Flutter/App.framework
 /Flutter/Flutter.framework
 /Flutter/Generated.xcconfig