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

pkg/compiler/lib/src/mirrors_used.dart

Index: pkg/compiler/lib/src/mirrors_used.dart
diff --git a/pkg/compiler/lib/src/mirrors_used.dart b/pkg/compiler/lib/src/mirrors_used.dart
index 0c05dd89de4e672ca178fec27396c2721ac3f49b..05f0720e63f906fe17b8cdf24ecdbb9a510121e6 100644
--- a/pkg/compiler/lib/src/mirrors_used.dart
+++ b/pkg/compiler/lib/src/mirrors_used.dart
@@ -329,7 +329,7 @@ class MirrorUsageAnalyzer {
   /// Convert a [constant] to an instance of [MirrorUsage] using information
   /// that was resolved during [MirrorUsageAnalyzerTask.validate].
   MirrorUsage buildUsage(ConstructedConstantValue constant) {
-    Map<Element, ConstantValue> fields = constant.fieldElements;
+    Map<Element, ConstantValue> fields = constant.fields;
     VariableElement symbolsField = compiler.mirrorsUsedClass.lookupLocalMember(
         'symbols');
     VariableElement targetsField = compiler.mirrorsUsedClass.lookupLocalMember(