Placeholders, plurals, and selects

You can also include application values in a message with
special syntax that uses a placeholder to generate a method
instead of a getter.
A placeholder, which must be a valid Dart identifier name,
becomes a positional parameter in the generated method in the
AppLocalizations code. Define a placeholder name by wrapping
it in curly braces as follows:

"{placeholderName}"

Define each placeholder in the placeholders object in the app’s .arb file.
For example, to define a hello message with a userName parameter,
add the following to lib/l10n/app_en.arb:

Regenerate the AppLocalizations file. This adds a hello method call to
the AppLocalizations.of(context) object, and the method accepts
a parameter of type String; the hello method returns a string.

Implement this by replacing the code passed into Builder
with the following: