Null exception thrown in changeNotifier after it creates the object which should not make it null

[DartBot]

Originally opened as dart-lang/sdk#20536

This issue was originally filed by [email protected]

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What steps will reproduce the problem?

1. Execute code that invokes ChangeNotifier.changes
2. In certain cases, the following exception is produced:

The null object does not have a getter 'stream'.

NoSuchMethodError: method not found: 'stream'
Receiver: null
Arguments: []

STACKTRACE:

­0 Object.noSuchMethod (dart:core-patch/object_patch.dart:45)

­1 ChangeNotifier.changes (package:observe/src/change_notifier.dart:29:21)

For reference, line 24-30 from change_notifier.dart:

  Stream<List<ChangeRecord>> get changes {
    if (_changes == null) {
      _changes = new StreamController.broadcast(sync: true,
          onListen: observed, onCancel: unobserved);
    }
    return _changes.stream;
  }

What is the expected output? What do you see instead?
_changes should never be null, so no exception should be thrown.

What version of the product are you using?
Dartium Version 37.0.2062.0 (287872) (64-bit)

On what operating system?
Linux

What browser (if applicable)?
Dartium Version 37.0.2062.0 (287872) (64-bit)

Please provide any additional information below.

C1

[DartBot]

<img src="https://avatars.githubusercontent.com/u/8631949?v=3" align="left" width="48" height="48"hspace="10"> Comment by floitschG

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Added Area-Pkg, Pkg-Observe, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/984921?v=3" align="left" width="48" height="48"hspace="10"> Comment by jakemac53

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Hmm, this is really weird. Is it possible for you to attach a repro of this?

Technically, StreamController.broadcast is a factory method and as such could return a null I believe (although this would be bad practice imo since factories and constructors appear identical in dart). However in reality it looks like it does not do that...