Change the badly named RegExp.pattern to return the original source.

tests/corelib/reg_exp_pattern_test.dart

+// Copyright (c) 2014, 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.
+// Dart test for testing regular expressions in Dart.
+
+import "package:expect/expect.dart";
+
+// Regression test for http://dartbug.com/17998
+
+main() {
+  for (var s in [
+         r"a",
+         r"a|b",
+         r"(?:)",
+         r"^",
+         r"$",
+         r"^$",
+         r"$^",
+         r"",
+         r"\\",
+         r"/",
+         r"[^]",
+         "\x00",
+      ]) {
+    Expect.equals(s, new RegExp(s).pattern);
+  }
+}