Use summary IDL file for interface classes.

pkg/analyzer/test/src/summary/prelinker_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.prelinker_test;
 
 import 'package:analyzer/dart/element/element.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/prelink.dart';
 import 'package:unittest/unittest.dart';
 
 import '../../reflective_tests.dart';
 import 'summarize_elements_test.dart';
 import 'summary_common.dart';
 
 main() {
   groupSep = ' | ';
   runReflectiveTests(PrelinkerTest);
@@ skipping 62 matching lines @@
         fail('Prelinker unexpectedly requested namespace for "$relativeUri"'
             ' (resolves to "$absoluteUri").'
             '  Namespaces available: ${uriToPublicNamespace.keys}');
       }
       return namespace;
     }
     linked = new LinkedLibrary.fromBuffer(
         prelink(unlinkedUnits[0], getPart, getImport).toBuffer());
   }
 }