hope for do expression

I think `do expression` is very useful in flutter's html-like code.

And `do expression` shouldn't be just anonymous function calling even though their code length are very near, because using anonymous function calling, the generator status can not pass down.

please have a look at https://github.com/tc39/proposal-do-expressions/issues/37

Start by reading this issue and the linked TC39 do-expressions proposal discussion, especially issue 37. Determine whether Dart should support do expressions and what generator behavior and Flutter-like syntax requirements would need to be specified. Done means a reviewed language-design decision with the required specification changes identified.

Written by the indexing model from the issue text.