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

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

Index: pkg/compiler/lib/src/native/behavior.dart
diff --git a/pkg/compiler/lib/src/native/behavior.dart b/pkg/compiler/lib/src/native/behavior.dart
index 7088f103a192c13dd66c167ecafc4a17d16a9909..6e6c53a8e357c9ace885c2333992ee334e8ae04c 100644
--- a/pkg/compiler/lib/src/native/behavior.dart
+++ b/pkg/compiler/lib/src/native/behavior.dart
@@ -694,14 +694,14 @@ class NativeBehavior {
       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].isString) {
+      if (fields.length != 1 || !fields.single.isString) {
         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();
       for (final typeString in specString.split('|')) {
         var type = _parseType(typeString, compiler, lookup, annotation);