Adjust expectations of library_env_test to take into account noopt, precompiled and product mode.

tests/language/library_env_test.dart

 // Copyright (c) 2015, 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';
 
 main() {
-  const NOT_PRESENT = null;
+  const NOT_PRESENT = false;
 
   Expect.isTrue(const bool.fromEnvironment("dart.library.async"));
   Expect.isTrue(const bool.fromEnvironment("dart.library.collection"));
   Expect.isTrue(const bool.fromEnvironment("dart.library.convert"));
   Expect.isTrue(const bool.fromEnvironment("dart.library.core"));
   Expect.isTrue(const bool.fromEnvironment("dart.library.typed_data"));
   Expect.isTrue(const bool.fromEnvironment("dart.library.developer"));
 
   // Internal libraries should not be exposed.
   Expect.equals(NOT_PRESENT,
@@ skipping 63 matching lines @@
       const bool.fromEnvironment("dart.library.converT",
           defaultValue: NOT_PRESENT));
   Expect.equals(NOT_PRESENT,
       const bool.fromEnvironment("dart.library.",
           defaultValue: NOT_PRESENT));
   Expect.equals(NOT_PRESENT,
       const bool.fromEnvironment("dart.library.core ",
           defaultValue: NOT_PRESENT));
 
 }