Use summary IDL file for interface classes.

pkg/analyzer/test/src/summary/summarize_elements_test.dart

 // Copyright (c) 2016, 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 analyzer.test.src.summary.summarize_elements_test;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/source_io.dart';
 import 'package:analyzer/src/summary/format.dart';
+import 'package:analyzer/src/summary/idl.dart';
 import 'package:analyzer/src/summary/public_namespace_computer.dart'
     as public_namespace;
 import 'package:analyzer/src/summary/summarize_elements.dart'
     as summarize_elements;
 import 'package:unittest/unittest.dart';
 
 import '../../generated/resolver_test.dart';
 import '../../reflective_tests.dart';
 import 'summary_common.dart';
 
@@ skipping 124 matching lines @@
       } else {
         UnlinkedPublicNamespace namespace =
             computePublicNamespaceFromText(text, source);
         expect(canonicalize(namespace),
             canonicalize(unlinkedUnits[i].publicNamespace),
             reason: 'publicNamespace(${unitUris[i]})');
       }
     }
   }
 }