Implement fromEnvironment on bool, int, String in dart2js.

tests/corelib/string_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=a -Db=bb -Dc=ccc
+
+import "package:expect/expect.dart";
+
+main(List<String> args) {
+  Expect.equals('a', const String.fromEnvironment('a'));
+  Expect.equals('bb', const String.fromEnvironment('b'));
+  Expect.equals('ccc', const String.fromEnvironment('c'));

[Lasse Reichstein Nielsen, 2013/10/30 13:34:48]

Do mixed tests as well:

With -Da=true -Db=0x10

Expect.equals('true', const String.fromEnvironment('a'));
Expect.equals(true, const bool.fromEnvironment('a'));
Expect.equals('0x10', const String.fromEnvironment('b'));
Expect.equals(16, const int.fromEnvironment('b'));
Expect.equals(false, const bool.fromEnvironment('b'));

[Søren Gjesse, 2013/10/30 13:59:14]

Updated tests in https://codereview.chromium.org/50983002

+}