Use summary IDL file for interface classes.

pkg/analyzer/lib/src/summary/summarize_elements.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 serialization.elements;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/type.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/resolver.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart';
 import 'package:analyzer/src/summary/format.dart';
+import 'package:analyzer/src/summary/idl.dart';
 import 'package:analyzer/src/summary/name_filter.dart';
 import 'package:analyzer/src/summary/summarize_const_expr.dart';
 
 /**
  * Serialize all the elements in [lib] to a summary using [ctx] as the context
  * for building the summary, and using [typeProvider] to find built-in types.
  */
 LibrarySerializationResult serializeLibrary(
     LibraryElement lib, TypeProvider typeProvider, bool strongMode) {
   var serializer = new _LibrarySerializer(lib, typeProvider, strongMode);
@@ skipping 1128 matching lines @@
       exportNames.add(new LinkedExportNameBuilder(
           name: name,
           dependency: serializeDependency(dependentLibrary),
           unit: unit,
           kind: kind));
     }
     pb.exportNames = exportNames;
     return pb;
   }
 }