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

sdk/lib/_internal/compiler/implementation/helpers/expensive_set.dart

Index: sdk/lib/_internal/compiler/implementation/helpers/expensive_set.dart
diff --git a/sdk/lib/_internal/compiler/implementation/helpers/expensive_set.dart b/sdk/lib/_internal/compiler/implementation/helpers/expensive_set.dart
index 61bad617677d4270d29ba06762a4553c8260fb18..55741afdf08974c974dcc9f4cb9b672887d37ae9 100644
--- a/sdk/lib/_internal/compiler/implementation/helpers/expensive_set.dart
+++ b/sdk/lib/_internal/compiler/implementation/helpers/expensive_set.dart
@@ -120,5 +120,17 @@ class ExpensiveSet<E> extends IterableBase<E> implements Set<E> {
     retainWhere(retainSet.contains);
   }
 
+  Set<E> toSet() {
+    var result = new ExpensiveSet<E>(_sets.length);
+    for (int i = 0; i < _sets.length; i++) {
+      result._sets[i] = _sets[i].toSet();
+    }
+    return result;
+  }
+
+  Set<E> cloneEmpty() {
+    return new ExpensiveSet<E>(_sets.length)
+  }
+
   String toString() => "expensive(${_sets[0]}x${_sets.length})";
 }