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

Fixes #27573
BUG= http://dartbug.com/27573
R=lrn@google.com

Review URL: https://codereview.chromium.org/2413233002 .

CHANGELOG.md

@@ -1,4 +1,7 @@
 ## 1.21.0
+### Core library changes
+
+* `dart:core`: `Set.difference` now takes a `Set<Object>` as argument.
 
 ### Language
 

pkg/compiler/lib/src/helpers/debug_collection.dart

@@ -261,7 +261,7 @@ class DebugSet<E> extends DebugIterable<E> implements Set<E> {
 
   Set<E> union(Set<E> other) => set.union(other);
 
-  Set<E> difference(Set<E> other) => set.difference(other);
+  Set<E> difference(Set<Object> other) => set.difference(other);
 
   void clear() => set.clear();
 

pkg/compiler/lib/src/helpers/expensive_set.dart

@@ -101,7 +101,7 @@ class ExpensiveSet<E> extends IterableBase<E> implements Set<E> {
     return _newSet()..addAll(this)..addAll(other);
   }
 
-  Set<E> difference(Set<E> other) {
+  Set<E> difference(Set<Object> other) {
     Set<E> result = _newSet();
     for (E element in this) {
       if (!other.contains(element)) result.add(element);

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

@@ -32,7 +32,7 @@ class ImmutableEmptySet<E> extends IterableBase<E> implements Set<E> {
 
   Set<E> union(Set<E> other) => new Set.from(other);
   Set<E> intersection(Set<E> other) => this;
-  Set<E> difference(Set<E> other) => this;
+  Set<E> difference(Set<Object> other) => this;
   Set<E> toSet() => new Set();
 }
 

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() {

sdk/lib/core/set.dart

@@ -185,7 +185,7 @@ abstract class Set<E> extends Iterable<E> implements EfficientLength {
    * That is, the returned set contains all the elements of this [Set] that
    * are not elements of [other] according to `other.contains`.
    */
-  Set<E> difference(Set<E> other);
+  Set<E> difference(Set<Object> other);
 
   /**
    * Removes all elements in the set.

tests/corelib/set_test.dart

@@ -166,6 +166,17 @@ void testInts(Set create()) {
   }
   Expect.isTrue(twice.difference(thrice).difference(twice).isEmpty);
 
+  // Test Set.difference with non-element type.
+  Set diffSet = create()..addAll([0, 1, 2, 499, 999]);
+  Set<Object> objectSet = new Set<Object>();
+  objectSet.add("foo");
+  objectSet.add(499);
+  Set diffResult = diffSet.difference(objectSet);
+  Expect.equals(4, diffResult.length);
+  for (int value in [0, 1, 2, 999]) {
+    Expect.isTrue(diffResult.contains(value));
+  }
+
   // Test Set.addAll.
   List list = new List(10);
   for (int i = 0; i < 10; i++) {