strong mode should allow type promotion from dynamic

[jmesserly]

@leafpetersen found this example:

String from(value) {
  if (value is Iterable) {
    List<String> result = [];
    var items = value.map((x) => from(x)).toList();
    result.addAll(items);
  }
  return "";
}
// gives:
// [warning] items (dynamic) will need runtime check to cast to type Iterable<String> (scratch.dart, line 5, col 19)

If we change the definition of from to be String from(Object value) you'll get a better type for items (List<dynamic>)

[jmesserly]

https://codereview.chromium.org/1585323003/
hardest part of that CL was the fact that it invalidated one of my old tests :)