Move the handling of operator[] into the new interceptors.

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2012, 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 mock_compiler;
 
 import 'dart:uri';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart' as api;
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart'
@@ skipping 33 matching lines @@
   boolConversionCheck(x) {}
   abstract class JavaScriptIndexingBehavior {}
   class JSInvocationMirror {}
   S() {}
   assertHelper(a){}
   throwNoSuchMethod(obj, name, arguments, expectedArgumentNames) {}''';
 
 const String DEFAULT_INTERCEPTORSLIB = r'''
   class JSArray {
     var length;
+    operator[](index) {}
   }
   class JSString {
     var length;
   }
   class JSNumber {
   }
   class JSInt {
   }
   class JSDouble {
   }
@@ skipping 208 matching lines @@
   operator []=(Node node, Element element) {
     map[node] = element;
   }
 
   operator [](Node node) => map[node];
 
   void remove(Node node) {
     map.remove(node);
   }
 }