code: @VisibleForTesting
    static ImmutableList<Fact> formatExpectedAndActual(String expected, String actual) {
      ImmutableList<Fact> result;
  
      // TODO(cpovirk): Call attention to differences in trailing whitespace.
      // TODO(cpovirk): And changes in the *kind* of whitespace characters in the middle of the line.
  
      result = Platform.makeDiff(expected, actual);
      if (result != null) {
        return result;
      }
  
      result = removeCommonPrefixAndSuffix(expected, actual);
      if (result != null) {
        return result;
      }
  
      return ImmutableList.of(fact("expected", expected), fact("but was", actual));
    }
docs: Returns one or more facts describing the difference between the given expected and actual
  values.
  
  <p>Currently, that means either 2 facts (one each for expected and actual) or 1 fact with a
  diff-like (but much simpler) view.
  
  <p>In the case of 2 facts, the facts contain either the full expected and actual values or, if
  the values have a long prefix or suffix in common, abbreviated values with "…" at the beginning
  or end.