Dart VM List.add should return void but returns int (array size)

[DartBot]

This issue was originally filed by @butlermatt

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Dart library reference indicates that List.add has a void return type. However it currently returns the new array size in the Dart VM. dartboard shows correct results.

eg:

main() {
  var list = new List();
  var x = list.add('foo');
  print(x);
}

Output: 1

Expected null.

[dgrove]

Added Area-VM, Triaged labels.

Set owner to @sgmitrovic.

Added Fixed label.