Implement fromEnvironment on bool, int and String

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
 
 import "package:expect/expect.dart";
 
-class A {
-  call(a) => a is num;
+main() {
+  Expect.isTrue(const bool.fromEnvironment('a'));
+  Expect.isFalse(const bool.fromEnvironment('b'));
 }
-
-main() {
-  Expect.isTrue(new A().call(42));
-  Expect.isFalse(new A()('foo'));
-}