Implement fromEnvironment on bool, int and String

tests/standalone/from_environment_string_script.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 "dart:convert";
+import "dart:io";
+
+main(List<String> args) {
+  var result = new Map();
+  result['a'] = const String.fromEnvironment('a', defaultValue: null);
+  result['b'] = const String.fromEnvironment('b');
+  result['c'] = const String.fromEnvironment('c', defaultValue: "default");
+  stdout.write(JSON.encode(result));
+}