Fix bug in our SSA optimizations: by giving an empty type to a box, we were assuming that anything …

tests/language/range_analysis2_test.dart

+// Copyright (c) 2013, 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 that used to remove bounds checks on
+// boxed variables.
+
+import "package:expect/expect.dart";
+
+main() {
+  var a = 0;
+  var b = [1];
+  foo() => b[a--] + b[a];
+  Expect.throws(foo, (e) => e is RangeError);
+}