Implement fromEnvironment on bool, int and String

tests/corelib/string_env_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.
 
 import "package:expect/expect.dart";
 
-class Foo {
-  var closures = {
-    'a': (x, y) => x + y
-  };
-}
-
 main() {
-  var closures = new Foo().closures;
-  Expect.equals(6, closures['a'](4, 2));
-}
+  Expect.equals("theDefault",
+               const String.env("NOT_IN_ENV", defaultValue:"theDefault"));
+  Expect.equals("", const String.env("NOT_IN_ENV"));
+}