Avoid dynamic calls, especially dynamic calls to collection getters/methods

[mkustermann]

To illustrate the point, let's look at code from BuiltListMultiMap:

  factory BuiltListMultimap([multimap = const {}]) {
    if (multimap is _BuiltListMultimap &&
        multimap.hasExactKeyAndValueTypes(K, V)) {
      return multimap as BuiltListMultimap<K, V>;
    } else if (multimap is Map) {
      return _BuiltListMultimap<K, V>.copy(multimap.keys, (k) => multimap[k]);
    } else if (multimap is BuiltListMultimap) {
      return _BuiltListMultimap<K, V>.copy(multimap.keys, (k) => multimap[k]);
    } else {
      return _BuiltListMultimap<K, V>.copy(multimap.keys, (k) => multimap[k]);
    }
  }

Here the parameter is typed dynamic multimap = const {}. As a result, the multimap.keys in the last else {} block is a dynamic get:keys call.

Now some applications may make classes implement the Map interface (e.g. proto classes with map mixins is common in g3).

=> This can now lead to thousands of small dynamic getter functions in production builds of an app.
=> Typing the parameter as Map multimap would avoid this
=> Similar dynamic calls are also in other places in the package:built_collection

/cc @davidmorgan

[dave26199]

Thanks for filing, happy to look at improvements here.

There are awkward points around Map in particular because the SDK does not have a read-only superinterface of Map with "keys" and "operator[]". It would help to add such an interface, it would allow removal of dynamic in some cases in a mostly non-breaking way. List and Set use Iterable so they don't have the same problem.

This is a very widely used package so breaking changes are hard to land, we'll have to look at what's feasible.

Other less than ideal typing is because the APIs are from before Dart 2 :) I've wanted to fix those for a while but it's hard to do.