Handle constant ?? expressions in constructors.

pkg/compiler/lib/src/serialization/element_serialization.dart

 // Copyright (c) 2015, 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 dart2js.serialization.elements;
 
 import '../common.dart';
 import '../common/names.dart';
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
@@ skipping 275 matching lines @@
     encoder.setUri(
         Key.CANONICAL_URI, element.canonicalUri, element.canonicalUri);
     encoder.setString(Key.LIBRARY_NAME, element.libraryName);
     SerializerUtil.serializeMembers(getMembers(element), encoder);
     encoder.setElement(Key.COMPILATION_UNIT, element.entryCompilationUnit);
     encoder.setElements(Key.COMPILATION_UNITS, getCompilationUnits(element));
     encoder.setElements(Key.IMPORTS, getImports(element));
     encoder.setElements(Key.EXPORTS, element.exports);
 
     encoder.setElements(Key.IMPORT_SCOPE, getImportedElements(element));
-
     encoder.setElements(Key.EXPORT_SCOPE, getExportedElements(element));
   }
 }
 
 class CompilationUnitSerializer implements ElementSerializer {
   const CompilationUnitSerializer();
 
   SerializedElementKind getSerializedKind(Element element) {
     if (element.isCompilationUnit) {
       return SerializedElementKind.COMPILATION_UNIT;
@@ skipping 535 matching lines @@
         return new LocalVariableElementZ(decoder);
       case SerializedElementKind.EXTERNAL_LIBRARY:
       case SerializedElementKind.EXTERNAL_LIBRARY_MEMBER:
       case SerializedElementKind.EXTERNAL_CLASS_MEMBER:
       case SerializedElementKind.EXTERNAL_CONSTRUCTOR:
         break;
     }
     throw new UnsupportedError("Unexpected element kind '${elementKind}.");
   }
 }