Always normalize a Range. The analysis does not work when it sees a range with unknown as bounds.

tests/language/value_range3_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.
+
+class A {
+  copy(array, index1, index2) {
+    if (index1 < index2 + index2) {
+      // dart2js used to remove the bounds check.
+      return array[index1];
+    }
+  }
+}
+
+main() {
+  Expect.throws(() => new A().copy(new List(0), 0, 1), (e) => e is RangeError);
+}