Change Set.toSet to always return a set with the same behavior.

pkg/third_party/html5lib/lib/src/css_class_set.dart

Index: pkg/third_party/html5lib/lib/src/css_class_set.dart
diff --git a/pkg/third_party/html5lib/lib/src/css_class_set.dart b/pkg/third_party/html5lib/lib/src/css_class_set.dart
index 03136a7877f139a3a6da0b88cef1f32905ab7447..daf29765730086076fdf5a25c6d18d7272daadee 100644
--- a/pkg/third_party/html5lib/lib/src/css_class_set.dart
+++ b/pkg/third_party/html5lib/lib/src/css_class_set.dart
@@ -279,6 +279,8 @@ abstract class CssClassSetImpl implements CssClassSet {
   Set<String> difference(Set<String> other) =>
     readClasses().difference(other);
 
+  Set<E> cloneEmpty() => new LinkedHashSet<String>();

[Lasse Reichstein Nielsen, 2014/05/19 07:30:05]

@jmesserly:
What does it mean that this is in pkg/third_party?
I expected pkg to be the packages we are maintaining as part of the repository,
so what does third_party mean here? Is this maintained in another repository
somewhere?
(I.e., should I revert the change or should I update the pubspec version?)

[Jennifer Messerly, 2014/05/19 17:44:42]

it means it's under a different license (MIT) and has different authors (listed
in pkg/third_party/html5lib/LICENSE).

IIRC, third_party was advice from the lawyers way back. we tried
"pkg/third_party" vs "third_party/pkg" but it's still confusing. kevmoo was
going to follow up and see if we can do something different. Sorry about the
confusion ... confuses me all the time too :-)

Not sure if we were tracking it, so I opened a bug for Kevin:
https://code.google.com/p/dart/issues/detail?id=18883

+
   String get first => readClasses().first;
   String get last => readClasses().last;
   String get single => readClasses().single;