type promotion stops working in some cases with inference

[jmesserly]

I'm running this on the YAML package.

dartanalyzer --strong issues the following warning, which is an error in DDC:

[warning] The getter 'name' is not defined for the class 'Token' (yaml/lib/src/parser.dart, line 257, col 47)

In this code:

  Event _parseNode({bool block: false, bool indentlessSequence: false}) {
    var token = _scanner.peek();

    if (token is AliasToken) {
      _scanner.scan();
      _state = _states.removeLast();
      return new AliasEvent(token.span, token.name); // <-- WARNING IS HERE
    }

Where _scanner.peek() returns a Token and AliasToken is defined as:

class AliasToken implements Token {
  get type => TokenType.ALIAS;
  final FileSpan span;

  /// The name of the anchor.
  final String name;

  AliasToken(this.span, this.name);

  String toString() => "ALIAS $name";
}

It looks like dynamic token = promotes to AliasToken but Token token = does not, even in existing analyzer. Not sure why, AliasToken should be a subtype of Token... @vsmenon @leafpetersen thoughts?

[jmesserly]

Sigh, I guess this is "not a bug" because of some mutation-in-closure that happens later in the method. And at least as currently spec'd type promotion is not smart enough to know that the closure it's afraid of doesn't even exist yet... Sigh.

[jmesserly]

I rolled this into #274