Mark dart:mirrors as having a (slightly) unstable API in the documentation.

sdk/lib/_internal/libraries.dart

 // Copyright (c) 2012, 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.
 
 library libraries;
 
 /**
  * A bit flag used by [LibraryInfo] indicating that a library is used by dart2js
  */
 const int DART2JS_PLATFORM = 1;
@@ skipping 57 matching lines @@
       dart2jsPath: "indexed_db/dart2js/indexed_db_dart2js.dart"),
 
   "io": const LibraryInfo(
       "io/io.dart",
       category: "Server",
       maturity: Maturity.STABLE,
       dart2jsPatchPath: "_internal/lib/io_patch.dart"),
 
   "isolate": const LibraryInfo(
       "isolate/isolate.dart",
-      maturity: Maturity.UNSTABLE,
+      maturity: Maturity.STABLE,
       dart2jsPatchPath: "_internal/lib/isolate_patch.dart"),
 
   "js": const LibraryInfo(
       "js/dartium/js_dartium.dart",
       category: "Client",
       maturity: Maturity.STABLE,
       dart2jsPath: "js/dart2js/js_dart2js.dart"),
 
   "math": const LibraryInfo(
       "math/math.dart",
@@ skipping 202 matching lines @@
     "The library is stable. API backwards-compatibility is guaranteed.\n"
     "However implementation details might change.");
 
   static const Maturity LOCKED = const Maturity(5, "Locked",
     "This library will not change except when serious bugs are encountered.");
 
   static const Maturity UNSPECIFIED = const Maturity(-1, "Unspecified",
     "The maturity for this library has not been specified.");
 }