Re-apply "Move the handling of operator[] into the new interceptors."

sdk/lib/_internal/compiler/implementation/js_backend/emitter.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.
 
 part of js_backend;
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
@@ skipping 1025 matching lines @@
       parameters.add(new js.Parameter('receiver'));
 
       String name;
       if (selector.isGetter()) {
         name = backend.namer.getterName(selector.library, selector.name);
       } else if (selector.isSetter()) {
         name = backend.namer.setterName(selector.library, selector.name);
         parameters.add(new js.Parameter('value'));
         arguments.add(new js.VariableUse('value'));
       } else {
-        assert(selector.isCall() || selector.isOperator());
         name = backend.namer.instanceMethodInvocationName(
             selector.library, selector.name, selector);
         for (int i = 0; i < selector.argumentCount; i++) {
           String argName = 'a$i';
           parameters.add(new js.Parameter(argName));
           arguments.add(new js.VariableUse(argName));
         }
       }
       js.Fun function =
           new js.Fun(parameters,
@@ skipping 963 matching lines @@
 """;
 const String HOOKS_API_USAGE = """
 // The code supports the following hooks:
 // dartPrint(message)   - if this function is defined it is called
 //                        instead of the Dart [print] method.
 // dartMainRunner(main) - if this function is defined, the Dart [main]
 //                        method will not be invoked directly.
 //                        Instead, a closure that will invoke [main] is
 //                        passed to [dartMainRunner].
 """;