The build methods of widgets are often a single
expression with multiple levels of nested constructors,
written using a strictly declarative subset of Dart.
Such nested expressions could be mechanically transformed
to or from any suitably expressive markup language.
For example, the UserAccountsDrawerHeader
widget has a long build method (20+ lines),
consisting of a single nested expression.
This can also be combined with the imperative style to build UIs
that would be harder to describe in a pure-declarative approach.