Fix some errors around []= in core types.

pkg/dev_compiler/tool/input_sdk/private/interceptors.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 dart._interceptors;
 
 import 'dart:collection';
 import 'dart:_internal' hide Symbol;
 import 'dart:_js_helper';
 import 'dart:_foreign_helper' show JS;
@@ skipping 26 matching lines @@
   int get hashCode => this ? (2 * 3 * 23 * 3761) : (269 * 811);
 
   Type get runtimeType => bool;
 }
 
 /**
  * The supertype for JSString and JSArray. Used by the backend as to
  * have a type mask that contains the objects that we can use the
  * native JS [] operator and length on.
  */
-abstract class JSIndexable {
+abstract class JSIndexable<E> {
   int get length;
-  operator[](int index);
+  E operator[](int index);
 }
 
 /**
- * The supertype for JSMutableArray and
- * JavaScriptIndexingBehavior. Used by the backend to have a type mask
- * that contains the objects we can use the JS []= operator on.
- */
-abstract class JSMutableIndexable extends JSIndexable {
-  operator[]=(int index, var value);
-}
-
-/**
  * The interface implemented by JavaScript objects.  These are methods in
  * addition to the regular Dart Object methods like [Object.hashCode].
  *
  * This is the type that should be exported by a JavaScript interop library.
  */
 abstract class JSObject {
 }
 
 
 /**
@@ skipping 33 matching lines @@
 
 // Obsolete in dart dev compiler. Added only so that the same version of
 // dart:html can be used in dart2js an dev compiler.
 // Warning: calls to these methods need to be removed before custom elements
 // and cross-frame dom objects behave correctly in ddc.
 // See https://github.com/dart-lang/dev_compiler/issues/517
 findInterceptorConstructorForType(Type type) { }
 findConstructorForNativeSubclassType(Type type, String name) { }
 getNativeInterceptor(object) {}
 setDispatchProperty(object, value) {}