StreamReader.ReadLine and returning a faked file line by lin

public static IEnumerable<string> ReadLines (this StreamReader source)
        {
            string line;
            const string COMMENT = @"//";

            if (source == null)
                throw new ArgumentNullException (String.Format ("Source stream '{0}' argument was 'null'", source));
            // skipp comments
            while ((line = source.ReadLine ()) != null)
                if (!line.StartsWith (COMMENT))
                    yield return line;
        }

  [Test, Isolated]
        public void ReadLines_ReadAllLinesOfAFile_ReturnsEachLineButNoCommentLines ()
        {
            Stream fakeStream = Isolate.Fake.Instance<Stream> ();
            StreamReader fakeReader = Isolate.Fake.Instance<StreamReader> ();

            Isolate.WhenCalled (() => fakeReader.ReadLine ()). ?????? What to do?????

            List<string> lines = new List<string> ();

            foreach (var line in ReadLines (fakeReader))
                lines.Add (line);

            Assert.AreEqual (lines[0], _fileContent[0]);
            Assert.AreEqual (lines[1], _fileContent[3]);
        }