We can only use JavaScript's String.split and String.+ if both inputs are Strings.

tests/corelib/string_operations_with_null_test.dart

Index: tests/corelib/string_operations_with_null_test.dart
===================================================================
--- tests/corelib/string_operations_with_null_test.dart	(revision 0)
+++ tests/corelib/string_operations_with_null_test.dart	(revision 0)
@@ -0,0 +1,19 @@
+// 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.
+
+import "package:expect/expect.dart";
+import "../language/compiler_annotations.dart";
+
+@DontInline()
+returnStringOrNull() {
+  () => 42;
+  return new DateTime.now().millisecondsSinceEpoch == 0 ? 'foo' : null;
+}
+
+main() {
+  Expect.throws(() => 'foo' + returnStringOrNull(),
+                (e) => e is ArgumentError);
+  Expect.throws(() => 'foo'.split(returnStringOrNull()),
+                (e) => e is ArgumentError || e is NoSuchMethodError);
+}