Change Set.difference API to accept Set<Object>.

pkg/compiler/lib/src/util/setlet.dart

Index: pkg/compiler/lib/src/util/setlet.dart
diff --git a/pkg/compiler/lib/src/util/setlet.dart b/pkg/compiler/lib/src/util/setlet.dart
index cd654c14627d2da25787175e164c07192b4efed1..c9d3fe53987a625c34e23caa0e4c77fca472e2f4 100644
--- a/pkg/compiler/lib/src/util/setlet.dart
+++ b/pkg/compiler/lib/src/util/setlet.dart
@@ -250,7 +250,7 @@ class Setlet<E> extends IterableBase<E> implements Set<E> {
   Setlet<E> intersection(Set<E> other) =>
       new Setlet<E>.from(this.where((e) => other.contains(e)));
 
-  Setlet<E> difference(Set<E> other) =>
+  Setlet<E> difference(Set<Object> other) =>
       new Setlet<E>.from(this.where((e) => !other.contains(e)));
 
   Setlet<E> toSet() {