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

pkg/compiler/lib/src/deferred_load.dart

Index: pkg/compiler/lib/src/deferred_load.dart
diff --git a/pkg/compiler/lib/src/deferred_load.dart b/pkg/compiler/lib/src/deferred_load.dart
index 243c3715933f9d0822f5eff9bcf3e75a7a578db1..b0192972f828aee0544a2007de1b73b2099d6034 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -513,7 +513,7 @@ class DeferredLoadTask extends CompilerTask {
               metadata.constant.value.getType(compiler.coreTypes).element;
           if (element == deferredLibraryClass) {
             ConstructedConstantValue constant = metadata.constant.value;
-            StringConstantValue s = constant.fields[0];
+            StringConstantValue s = constant.fields.values.single;
             result = s.primitiveValue.slowToString();
             break;
           }