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

pkg/compiler/lib/src/deferred_load.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library deferred_load;
 
 import 'constants/expressions.dart';
 import 'constants/values.dart' show
     ConstantValue,
     ConstructedConstantValue,
@@ skipping 495 matching lines @@
         result = import.prefix.toString();
       } else {
         Link<MetadataAnnotation> metadatas = import.metadata;
         assert(metadatas != null);
         for (MetadataAnnotation metadata in metadatas) {
           metadata.ensureResolved(compiler);
           Element element =
               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;
           }
         }
       }
       assert(result != null);
       importDeferName[import] = makeUnique(result, usedImportNames);;
     }
 
     int counter = 1;
@@ skipping 333 matching lines @@
         _importingLibrary = importingLibrary;
 
   String get importingLibraryName {
     String libraryName = _importingLibrary.getLibraryName();
     return libraryName == ""
       ? "<unnamed>"
       : libraryName;
   }
 
 }