@Test
  public void testSize() throws IOException {
    RegularFile file = regularFile(10);
    FileChannel channel = channel(file, READ);

    assertEquals(10, channel.size());

    file.write(10, new byte[90], 0, 90);
    assertEquals(100, channel.size());
  }
Most of the behavior of {@link JimfsFileChannel} is handled by the {@link RegularFile}
implementations, so the thorough tests of that are in {@link RegularFileTest}. This mostly tests
interactions with the file and channel positions.

@author Colin Decker
jimfs/src/test/java/com/google/common/jimfs/JimfsFileChannelTest.java