fix corelib_2/iterable_to_set_test to reflect strong mode runtime checks

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 27 matching lines @@
   Expect.isTrue(setCopy is Set<int>);
   Expect.isFalse(setCopy is Set<String>);
   Expect.isFalse(identical(setCopy, set1));
 
   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));
+  {
+    var setCopy = set3.toSet();

[Jennifer Messerly, 2017/08/04 23:33:11]

the setCopy local in the outer scope has inferred type Set<int>, so this
implicit downcast was failing.

[Bob Nystrom, 2017/08/04 23:45:41]

In other tests where we've had to split a reused variable, we tend to just
rename instead of introducing a block. How about "set3Copy"?

[Jennifer Messerly, 2017/08/05 00:05:43]

Done.

+    Expect.setEquals(set3, setCopy);
+    Expect.isFalse(setCopy is Set<String>);

[Jennifer Messerly, 2017/08/04 23:33:11]

these should be false, not true.

[Bob Nystrom, 2017/08/04 23:45:41]

Should we add an isTrue check for is Set<dynamic>?

[Jennifer Messerly, 2017/08/05 00:05:43]

Done.

+    Expect.isFalse(setCopy is Set<int>);
+    Expect.isFalse(identical(setCopy, set3));
+  }
 }