Implement fromEnvironment on bool, int and String

tests/corelib/string_from_environment_default_value.dart

Index: tests/corelib/string_from_environment_default_value.dart
diff --git a/tests/corelib/main_test.dart b/tests/corelib/string_from_environment_default_value.dart
similarity index 63%
copy from tests/corelib/main_test.dart
copy to tests/corelib/string_from_environment_default_value.dart
index 141281196fae792229f0fe203315105f3d164794..7e6547d01bd3a2aecf52cee9d4be6a522df1888e 100644
--- a/tests/corelib/main_test.dart
+++ b/tests/corelib/string_from_environment_default_value.dart
@@ -2,9 +2,10 @@
 // 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.
 
-library main_test;
 import "package:expect/expect.dart";
 
 main(List<String> args) {
-  Expect.equals(0, args.length);
+  Expect.isNull(const String.fromEnvironment('NOT_FOUND'));
+  Expect.equals('x',
+                const String.fromEnvironment('NOT_FOUND', defaultValue: 'x'));
 }