Fix some errors around []= in core types.

pkg/dev_compiler/tool/input_sdk/private/js_string.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 dart._interceptors;
 
 /**
  * The interceptor class for [String]. The compiler recognizes this
  * class as an interceptor, and changes references to [:this:] to
  * actually use the receiver of the method, which is generated as an extra
  * argument added to each member.
  */
 @JsPeerInterface(name: 'String')
-class JSString extends Interceptor implements String, JSIndexable {
+class JSString extends Interceptor implements String, JSIndexable<String> {
   const JSString();
 
   int codeUnitAt(int index) {
     if (index is !int) throw diagnoseIndexError(this, index);
     if (index < 0) throw diagnoseIndexError(this, index);
     if (index >= length) throw diagnoseIndexError(this, index);
     return JS('int', r'#.charCodeAt(#)', this, index);
   }
 
   Iterable<Match> allMatches(String string, [int start = 0]) {
@@ skipping 439 matching lines @@
   Type get runtimeType => String;
 
   int get length => JS('int', r'#.length', this);
 
   String operator [](int index) {
     if (index is !int) throw diagnoseIndexError(this, index);
     if (index >= length || index < 0) throw diagnoseIndexError(this, index);
     return JS('String', '#[#]', this, index);
   }
 }