Retain elements a finer granularity than library.

dart/sdk/lib/_internal/compiler/implementation/js_backend/backend.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;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked;
 
   JavaScriptItemCompilationContext()
@@ skipping 1444 matching lines @@
     } else {
       metadataGetOfStaticFunction.add(element);
     }
   }
 
   void registerMirrorUsage(Set<String> symbols,
                            Set<Element> targets,
                            Set<Element> metaTargets) {
     if (symbols != null) symbolsUsed.addAll(symbols);
     if (targets != null) {
-      for (Element element in targets) {
-        // TODO(ahe): Implement finer granularity.
-        targetsUsed.add(element.getLibrary());
+      for (Element target in targets) {
+        if (target.isAbstractField()) {
+          AbstractFieldElement field = target;
+          targetsUsed.add(field.getter);
+          targetsUsed.add(field.setter);
+        } else {
+          targetsUsed.add(target);
+        }
       }
     }
     if (metaTargets != null) metaTargetsUsed.addAll(metaTargets);
   }
 
   bool isNeededForReflection(Element element) {
     // TODO(ahe): Implement this.
     if (!metaTargetsUsed.isEmpty) return true;
-    if (targetsUsed.contains(element.getLibrary())) return true;
+    if (!targetsUsed.isEmpty) {
+      for (Element e = element; e != null; e = e.enclosingElement) {
+        if (targetsUsed.contains(e)) return true;
+      }
+    }
     return false;
   }
 }
 
 /// Records that [type] is used by [user.element].
 class Dependency {
   final DartType type;
   final TreeElements user;
 
   const Dependency(this.type, this.user);
 }