Implement fromEnvironment on bool, int and String

tests/corelib/string_from_environment_test.dart

Index: tests/corelib/string_from_environment_test.dart
diff --git a/tests/corelib/string_from_environment_test.dart b/tests/corelib/string_from_environment_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..92cfca94b48e3bf00b3236cc45e028dad3c51e54
--- /dev/null
+++ b/tests/corelib/string_from_environment_test.dart
@@ -0,0 +1,16 @@
+// 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.
+// VMOptions=-Da=a -Db=bb -Dc=ccc -Dd=
+
+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'));
+  Expect.equals('', const String.fromEnvironment('d'));
+  Expect.isNull(const String.fromEnvironment('NOT_FOUND'));
+  Expect.equals('x',
+                const String.fromEnvironment('NOT_FOUND', defaultValue: 'x'));
+}