docs: function chain(value, fn, label) {
      _emberMetalDebug.assert('Must call chain with a label', !!label);
      if (isStream(value)) {
        var stream = new _emberMetalStreamsStream.Stream(fn, function () {
          return label + '(' + labelFor(value) + ')';
        });
        stream.addDependency(value);
        return stream;
      } else {
        return fn();
      }
    }