Migrated test block 12 to Dart 2.0.

tests/corelib_2/iterable_to_set_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
 
 main() {
   List<int> list1 = <int>[1, 2, 3];
   List<int> list2 = const <int>[4, 4];
   List<String> list3 = <String>[];
@@ skipping 10 matching lines @@
   Expect.isTrue(setCopy.contains(3));
   Expect.isTrue(setCopy is Set<int>);
   Expect.isFalse(setCopy is Set<String>);
 
   setCopy = list2.toSet();
   Expect.equals(1, setCopy.length);
   Expect.isTrue(setCopy.contains(4));
   Expect.isTrue(setCopy is Set<int>);
   Expect.isFalse(setCopy is Set<String>);
 
-  setCopy = list3.toSet();
-  Expect.isTrue(setCopy.isEmpty);
-  Expect.isTrue(setCopy is Set<String>);
-  Expect.isFalse(setCopy is Set<int>);
+  var setStrCopy = list3.toSet();
+  Expect.isTrue(setStrCopy.isEmpty);
+  Expect.isTrue(setStrCopy is Set<String>);
+  Expect.isFalse(setStrCopy is Set<int>);
 
   setCopy = set1.toSet();
   Expect.setEquals(set1, setCopy);
   Expect.isTrue(setCopy is Set<int>);
   Expect.isFalse(setCopy is Set<String>);
   Expect.isFalse(identical(setCopy, set1));
 
-  setCopy = set2.toSet();
-  Expect.setEquals(set2, setCopy);
-  Expect.isTrue(setCopy is Set<String>);
-  Expect.isFalse(setCopy is Set<int>);
-  Expect.isFalse(identical(setCopy, set2));
+  setStrCopy = set2.toSet();
+  Expect.setEquals(set2, setStrCopy);
+  Expect.isTrue(setStrCopy is Set<String>);
+  Expect.isFalse(setStrCopy is Set<int>);
+  Expect.isFalse(identical(setStrCopy, set2));
 
   setCopy = set3.toSet();
   Expect.setEquals(set3, setCopy);
   Expect.isTrue(setCopy is Set<String>);
   Expect.isTrue(setCopy is Set<int>);
   Expect.isFalse(identical(setCopy, set3));
 }