Add ability to generate list expressions

[filiph]

It's possible to build literal lists with literalList. But it's not possible to build a list of expressions.

Something like:

list(refer('a'), refer('b'));

I tried to build it myself, but I had to revert to building a string using DartEmitter. The classes that might help me do it the proper way (like BinaryExpression) are private.

This might be solved by an overarching fix, like adding a class that interleaves literal strings with expressions (see proposal in #223 (comment)) or some ability to plug in custom visitor methods.

[matanlurey]

I am on mobile but literalList supports arbitrary expressions or references as well.

The name literal just means the list literal syntax, i.e.:

listLiteral([refer('a'), refer('b')])

... should just work. Let me know if it doesn't!