...
  flutter:
    ...
    deferred-components:
    ...
    
    The flutter tool looks for the deferred-components
  entry in the pubspec.yaml to determine whether the
  app should be built as deferred or not.
  This can be left empty for now unless you already
  know the components desired and the Dart deferred libraries
  that go into each. You will fill in this section later
  in step 3.3 once gen_snapshot produces the loading units.

Step 2: Implementing deferred Dart libraries

Next, implement deferred loaded Dart libraries in your
app’s Dart code. The implementation does not need
to be feature complete yet. The example in the
rest of this page adds a new simple deferred widget
as a placeholder. You can also convert existing code
to be deferred by modifying the imports and
guarding usages of deferred code behind loadLibrary()
Futures.