[Null-aware elements] Implement AST enhancements

[chloestefantsova]

A new AST node should be added to represent the null-aware elements in list and set literals:

testList(int? x) => <int>[?x];
testSet(int? x) => <int>[?x];

The MapLiteralEntry node should be updated to contain the optional null-aware tokens for the key and the value:

testNullAwareKey(int? x, String y) => <int, String>{?x: y};
testNullAwareValue(int x, String? y) => <int, String>{x: ?y};
testBothNullAware(int? x, String? y) => <int, String>{?x: ?y};
testNoneNullAware(int x, String y) => <int, String>{x: y};