Optimizations on String.concat and String.toString need to make sure the receiver is not null.

tests/language/string_optimizations_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's type inferrer, that used to not
+// correclty infer optional named parameters.
+
+import "package:expect/expect.dart";
+import "compiler_annotations.dart";
+
+@DontInline()
+foo({path}) {
+  () => 42;
+  return path.toString();
+}
+
+@DontInline()
+bar({path}) {
+  () => 42;
+  return path;
+}
+
+main() {
+  var a = [foo(path: '42'), foo(), 42, bar(path: '54')];
+  Expect.isTrue(a[1] is String);
+  Expect.throws(() => bar().concat('54'), (e) => e is NoSuchMethodError);
+}