Use Map<FieldElement, ConstantValue> for fields in ConstructedConstantValue.

pkg/compiler/lib/src/native/enqueue.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.
 
 part of native;
 
 /**
  * This could be an abstract class but we use it as a stub for the dart_backend.
  */
 class NativeEnqueuer {
@@ skipping 306 matching lines @@
     ClassElement annotationClass = annotationJsNameClass;
     for (Link<MetadataAnnotation> link = element.metadata;
          !link.isEmpty;
          link = link.tail) {
       MetadataAnnotation annotation = link.head.ensureResolved(compiler);
       ConstantValue value = annotation.constant.value;
       if (!value.isConstructedObject) continue;
       ConstructedConstantValue constructedObject = value;
       if (constructedObject.type.element != annotationClass) continue;
 
-      List<ConstantValue> fields = constructedObject.fields;
+      Iterable<ConstantValue> fields = constructedObject.fields.values;
       // TODO(sra): Better validation of the constant.
-      if (fields.length != 1 || fields[0] is! StringConstantValue) {
+      if (fields.length != 1 || fields.single is! StringConstantValue) {
         PartialMetadataAnnotation partial = annotation;
         compiler.internalError(annotation,
             'Annotations needs one string: ${partial.parseNode(compiler)}');
       }
-      StringConstantValue specStringConstant = fields[0];
+      StringConstantValue specStringConstant = fields.single;
       String specString = specStringConstant.toDartString().slowToString();
       if (name == null) {
         name = specString;
       } else {
         PartialMetadataAnnotation partial = annotation;
         compiler.internalError(annotation,
             'Too many JSName annotations: ${partial.parseNode(compiler)}');
       }
     }
     return name;
@@ skipping 338 matching lines @@
         superclass,
         () => <ClassElement>[]);
     directSubtypes.add(cls);
   }
 
   void logSummary(log(message)) {
     log('Compiled ${registeredClasses.length} native classes, '
         '${unusedClasses.length} native classes omitted.');
   }
 }