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

pkg/compiler/lib/src/native/enqueue.dart

Index: pkg/compiler/lib/src/native/enqueue.dart
diff --git a/pkg/compiler/lib/src/native/enqueue.dart b/pkg/compiler/lib/src/native/enqueue.dart
index 7d698b65f489d168b40cfe942e79d3d48d5b20aa..11968db304e8140d1c10b153992b1102770081c9 100644
--- a/pkg/compiler/lib/src/native/enqueue.dart
+++ b/pkg/compiler/lib/src/native/enqueue.dart
@@ -324,14 +324,14 @@ abstract class NativeEnqueuerBase implements NativeEnqueuer {
       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;