ObservableList.sublist does not return an ObservableList

[DartBot]

This issue was originally filed by @butlermatt

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This used to work correctly in web_ui, but as I port my project over to Polymer I'm finding that ObservableList.sublist() does not return an ObservableList. I instead have to wrap the return of sublist in toObservable.

eg:

// This throws an exception
ObservableList get items {
  if(_itemList.length < NUM_ROWS) return _itemList;
  return _itemList.sublist(0, NUM_ROWS);
}

Uncaught Error: type 'List' is not a subtype of type 'ObservableList' of 'function result'.

// This works
ObservableList get items {
  if(_itemList.length < NUM_ROWS) return _itemList;
  return toObservable(_itemList.sublist(0, NUM_ROWS));
}

[kasperl]

Added Area-Polymer, Triaged labels.

[sigmundch]

Added this to the Later milestone.

[sigmundch]

Removed this from the Later milestone.
Added this to the M9 milestone.

[clayberg]

Removed this from the M9 milestone.
Added this to the 1.1 milestone.

[sigmundch]

Removed this from the 1.1 milestone.
Added this to the 1.2 milestone.

[sigmundch]

Added Library-Observe label.

[anders-sandholm]

Removed Library-Observe label.
Added Pkg-Observe label.

[jmesserly]

we generally don't try to preserve observable-ness in all accessors, as that would greatly complicate ObservableList/Map impls. We'd need to know about every sublist, iterable, etc, and propagate change notifications. While it's a very neat idea and I support someone taking it on :-), we don't really have bandwidth. (and Object.observe in JS doesn't support it either, for similar reasons).

OTOH, if we fix https://code.google.com/p/dart/issues/detail?id=16342 it will essentially work again (using dirty checking like Web UI did).

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Added NotPlanned label.

[DartBot]

This issue has been moved to dart-archive/observe#45.