Fix checkedInstructionOrNonGenerateAtUseSite.

tests/language/issue15720_test.dart

+// Copyright (c) 2011, 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.
+
+// Regression test for dart2js, issue 15720.
+
+class B {}
+
+confuse(x) {
+  if (new DateTime.now() == 42) return confuse(x);
+  return x;
+}
+
+main() {
+  Set<B> set = new Set<B>.from([]);
+
+  confuse(499);
+  confuse(set);
+
+  // Dart2js used to reuse a variable name, overwriting the `set` variable with
+  // one of the B's.
+  var t1 = new B();
+  var t2 = new B();
+  var t3 = new B();
+  var t4 = new B();
+
+  set.addAll([t1, t2, t3, t4]);
+  confuse(7);
+
+  set.addAll([t1, t2, t3, t4]);
+}