Avoid counting custom elements as added due to mirrors

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;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 1881 matching lines @@
         staticFields.add(target);
       } else if (target.isLibrary || target.isClass) {
         addFieldsInContainer(target);
       }
     }
     return staticFields;
   }
 
   /// Called when [enqueuer] is empty, but before it is closed.
   void onQueueEmpty(Enqueuer enqueuer) {
+    // Add elements referenced only via custom elements.  Return early if any
+    // elements are added to avoid counting the elements as due to mirrors.
+    customElementsAnalysis.onQueueEmpty(enqueuer);
+    if (!enqueuer.queueIsEmpty) return;
+
     if (!enqueuer.isResolutionQueue && preMirrorsMethodCount == 0) {
       preMirrorsMethodCount = generatedCode.length;
     }
 
     if (isTreeShakingDisabled) {
       enqueuer.enqueueEverything();
     } else if (!targetsUsed.isEmpty && enqueuer.isResolutionQueue) {
       // Add all static elements (not classes) that have been requested for
       // reflection. If there is no mirror-usage these are probably not
       // necessary, but the backend relies on them being resolved.
       enqueuer.enqueueReflectiveStaticFields(_findStaticFieldTargets());
     }
 
     if (mustPreserveNames) compiler.log('Preserving names.');
 
     if (mustRetainMetadata) {
       compiler.log('Retaining metadata.');
 
       compiler.libraries.values.forEach(retainMetadataOf);
       for (Dependency dependency in metadataConstants) {
         registerCompileTimeConstant(
             dependency.constant, dependency.registry);
       }
       metadataConstants.clear();
     }
-
-    customElementsAnalysis.onQueueEmpty(enqueuer);
   }
 
   void onElementResolved(Element element, TreeElements elements) {
     LibraryElement library = element.library;
     if (!library.isPlatformLibrary && !library.canUseNative) return;
     bool hasNoInline = false;
     bool hasNoThrows = false;
     bool hasNoSideEffects = false;
     for (MetadataAnnotation metadata in element.metadata) {
       metadata.ensureResolved(compiler);
@@ skipping 49 matching lines @@
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final Constant constant;
   // TODO(johnniwinther): Change to [Element] when dependency nodes are added.
   final Registry registry;
 
   const Dependency(this.constant, this.registry);
 }