Change bool.fromEnvironment to recognize "true" and "false" and use default for rest.

tests/corelib/bool_from_environment_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.
-// SharedOptions=-Da=true -Db=false
+// SharedOptions=-Da=true -Db=false -Dc=NOTBOOL -Dd=True
 
 import "package:expect/expect.dart";
 
 main() {
   Expect.isTrue(const bool.fromEnvironment('a'));
   Expect.isFalse(const bool.fromEnvironment('b'));
+  Expect.isTrue(const bool.fromEnvironment('c', defaultValue: true));
+  Expect.isFalse(const bool.fromEnvironment('c', defaultValue: false));
+  Expect.isFalse(const bool.fromEnvironment('d', defaultValue: false));
 }