Implement MirrorUsed.targets for libraries

dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.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.
 
 part of js_backend;
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
@@ skipping 708 matching lines @@
             js('supr = s[0]'),
             js('var mixin = collectedClasses[s[1]]'),
             js.forIn('d', 'mixin', [
               js.if_('hasOwnProperty.call(mixin, d)'
                      '&& !hasOwnProperty.call(desc, d)',
                 js('desc[d] = mixin[d]'))
             ]),
           ])),
 
           js('var constructor = defineClass(name, cls, fields, desc)'),
-          optional(backend.hasRetainedMetadata,
+          optional(backend.isTreeShakingDisabled,
                    js('constructor["${namer.metadataField}"] = desc')),
           js('isolateProperties[cls] = constructor'),
           js.if_('supr', js('pendingClasses[cls] = supr'))
         ])
       ]),
 
       js('var finishedClasses = {}'),
 
       buildFinishClass(),
     ];
@@ skipping 3303 matching lines @@
 
 const String HOOKS_API_USAGE = """
 // The code supports the following hooks:
 // dartPrint(message)   - if this function is defined it is called
 //                        instead of the Dart [print] method.
 // dartMainRunner(main) - if this function is defined, the Dart [main]
 //                        method will not be invoked directly.
 //                        Instead, a closure that will invoke [main] is
 //                        passed to [dartMainRunner].
 """;