Implement fromEnvironment on bool, int and String

tests/corelib/int_from_environment3_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.
+
+main(List<String> args) {
+  const int.fromEnvironment('NOT_FOUND', defaultValue: '');  /// 01: compile-time error
+  const int.fromEnvironment('NOT_FOUND', defaultValue: true);  /// 02: compile-time error
+  const int.fromEnvironment(null);  /// 03: compile-time error
+  const int.fromEnvironment(1);  /// 04: compile-time error
+  const int.fromEnvironment([]);  /// 05: compile-time error
+}