_DeepMatcher crash on badly behaved Iterable

[davidmorgan]

I don't have time to create a minimal repro right now, but writing this down so it doesn't get lost.

It looks like an iterable that doesn't always return the same results can cause a crash in expect(iterable, [1, 2, 3]).

  NoSuchMethodError: The getter 'describeProblem' was called on null.
  Receiver: null
  Tried calling: describeProblem
  dart:core                                                    Object.noSuchMethod
  package:matcher/src/equals_matcher.dart 273:36               _DeepMatcher.describeMismatch

[davidmorgan]

Found a repro; it's not about the iterable changing, it's about the iterable throwing.

expect([1].map((x) => throw 'whoops'), []);

-->

  NoSuchMethodError: The getter 'describeProblem' was called on null.
  Receiver: null
  Tried calling: describeProblem
  dart:core                                                                   Object.noSuchMethod
  package:matcher/src/equals_matcher.dart 273:36                              _DeepMatcher.describeMismatch