Fix resynthesis of operator[]= from summary.

pkg/analyzer/test/src/summary/resynthesize_test.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 test.src.serialization.elements_test;
 
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/summary/builder.dart';
 import 'package:analyzer/src/summary/format.dart';
@@ skipping 748 matching lines @@
   }
 
   test_method_parameter_return_type() {
     checkLibrary('class C { f(int g()) {} }');
   }
 
   test_method_parameter_return_type_void() {
     checkLibrary('class C { f(void g()) {} }');
   }
 
+  test_operator_index() {
+    checkLibrary('class C { bool operator[](int i) => null; }');
+  }
+
+  test_operator_index_set() {
+    checkLibrary('class C { void operator[]=(int i, bool v) {} }');
+  }
+
   test_parts() {
     addNamedSource('/a.dart', 'part of my.lib;');
     addNamedSource('/b.dart', 'part of my.lib;');
     checkLibrary('library my.lib; part "a.dart"; part "b.dart";');
   }
 
   test_setters() {
     checkLibrary('void set x(int value) {} set y(value) {}');
   }
 
@@ skipping 181 matching lines @@
   }
 
   test_variable_const() {
     checkLibrary('const int i = 0;');
   }
 
   test_variables() {
     checkLibrary('int i; int j;');
   }
 }