Add corelib tests

test/codegen/corelib/int_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=1 -Db=-12 -Dc=0x123 -Dd=-0x1234 -De=+0x112296 -Df=99999999999999999999
+
+import "package:expect/expect.dart";
+
+main() {
+  Expect.equals(1, const int.fromEnvironment('a'));
+  Expect.equals(-12, const int.fromEnvironment('b'));
+  Expect.equals(0x123, const int.fromEnvironment('c'));
+  Expect.equals(-0x1234, const int.fromEnvironment('d'));
+  Expect.equals(0x112296, const int.fromEnvironment('e'));
+  Expect.equals(99999999999999999999, const int.fromEnvironment('f'));
+}