Fix assertion failure in range analysis.

tests/language/vm/regress_21245_test.dart

+// Copyright (c) 2014, 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.
+// VMOptions=--optimization_counter_threshold=10 --no-use-osr
+
+test(a) {
+  var e;
+  for (var i = 0; i < a.length; i++) {
+    e = a[i];
+    for (var j = 0; j < i; j++) {
+      e = a[j];
+    }
+  }
+  return e;
+}
+
+main() {
+  var a = [0, 1, 2, 3, 4, 5];
+  for (var i = 0; i < 20; i++) {
+    test(a);
+  }
+}
+