Linter: set literals

[pq]

Meta-issue corresponding to dart-lang/language#37, the implementation of set literals.

• update prefer_collection_literals (Dart Synonym "Checking for empty string" typo #1413)
• update prefer_const_declarations to be aware of implicit const Set literals (Cyclic type variable test has a legal declaration flagged by the VM #1427)

[pq]

Minimally, we might update prefer_collection_literals to handle common cases (and ideally have corresponding assists).

Food for thought:

var s = new Set(); // LINT? <= see below
var s1 = new Set.from(['foo', 'bar', 'baz']); // LINT
var s3 = ['foo', 'bar', 'baz'].toSet(); // LINT

If I'm understanding the feature spec, I think we want to handle these too?

var s = new LinkedHashSet(); // LINT
var s1 = new LinkedHashSet.from(['foo', 'bar', 'baz']); // LINT

/cc @bwilkerson

[pq]

A nuance:

var s = new Set(); 

doesn't imply the simple literal:

var s = {}; 

(since that's actually a map 🤔.)

If a set literal is preferred in this case, I guess we have two options:

1. Set s = {};, or
2. var s = <dynamic>{};

@munificent: thinking ahead to readability, what would you suggest?

[munificent]

The latter is how Effective Dart suggests you write the code.

[pq]

Thanks!

[dballesteros7]

Is it intended that the linter already triggers on code like:

final x = Set<String>();

Even though Set literals are not yet released? See this CI build:
https://travis-ci.org/dnfield/flutter_svg/builds/496594380

[pq]

Hi @dballesteros7. A linter with prefer_collection_literals updated to flag Set literals rolled into the SDK right after the feature was enabled by default. Since Flutter pulls SDK versions irrespective of official releases that might be the source of the confusion. (The feature should be enabled in that SDK though.)