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

pkg/compiler/lib/src/js_backend/constant_emitter.dart

Index: pkg/compiler/lib/src/js_backend/constant_emitter.dart
diff --git a/pkg/compiler/lib/src/js_backend/constant_emitter.dart b/pkg/compiler/lib/src/js_backend/constant_emitter.dart
index 8f5738f96e59a80118faf0807c066028ced1dee8..1b3e95fed7efa0b9fb1b0a6ff69d9d1e2ac6fc74 100644
--- a/pkg/compiler/lib/src/js_backend/constant_emitter.dart
+++ b/pkg/compiler/lib/src/js_backend/constant_emitter.dart
@@ -272,14 +272,15 @@ class ConstantEmitter
     Element element = constant.type.element;
     if (element.isForeign(backend)
         && element.name == 'JS_CONST') {
-      StringConstantValue str = constant.fields[0];
+      StringConstantValue str = constant.fields.values.single;
       String value = str.primitiveValue.slowToString();
       return new jsAst.LiteralExpression(stripComments(value));
     }
     jsAst.Expression constructor =
         backend.emitter.constructorAccess(constant.type.element);
     List<jsAst.Expression> fields =
-        constant.fields.map(constantReferenceGenerator).toList(growable: false);
+        constant.fields.values.map(constantReferenceGenerator)
+        .toList(growable: false);
     jsAst.New instantiation = new jsAst.New(constructor, fields);
     return maybeAddTypeArguments(constant.type, instantiation);
   }