Fix more errors in DDC SDK.

pkg/dev_compiler/tool/input_sdk/lib/js/dart2js/js_dart2js.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.
 
 /**
  * Support for interoperating with JavaScript.
  *
  * This library provides access to JavaScript objects from Dart, allowing
  * Dart code to get and set properties, and call methods of JavaScript objects
  * and invoke JavaScript functions. The library takes care of converting
@@ skipping 186 matching lines @@
     }
     return _convertToDart(JS('', '#[#]', _jsObject, property));
   }
 
   /**
    * Sets the value associated with [property] on the proxied JavaScript
    * object.
    *
    * The type of [property] must be either [String] or [num].
    */
-  operator []=(Object property, value) {
+  void operator []=(Object property, value) {
     if (property is! String && property is! num) {
       throw new ArgumentError("property is not a String or num");
     }
     JS('', '#[#] = #', _jsObject, property, _convertToJS(value));
   }
 
   int get hashCode => 0;
 
   bool operator ==(other) =>
       other is JsObject && JS('bool', '# === #', _jsObject, other._jsObject);
@@ skipping 142 matching lines @@
 
   E operator [](Object index) {
     // TODO(justinfagnani): fix the semantics for non-ints
     // dartbug.com/14605
     if (index is num && index == index.toInt()) {
       _checkIndex(index);
     }
     return super[index] as E;
   }
 
-  void operator []=(Object index, E value) {
+  void operator []=(Object index, value) {

[Bob Nystrom, 2017/05/10 22:53:59]

Removing "E" here means that the argument is no longer checked to see if it
conforms to the type argument. Is that OK? Should this be a covariant override
instead?

     // TODO(justinfagnani): fix the semantics for non-ints
     // dartbug.com/14605
     if (index is num && index == index.toInt()) {
       _checkIndex(index);
     }
     super[index] = value;
   }
 
   int get length {
     // Check the length honours the List contract.
@@ skipping 199 matching lines @@
         '  for (let arg of arguments) {'
         '    args.push(arg);'
         '  }'
         '  return #(...args);'
         '}',
         f);
     _interopCaptureThisExpando[f] = ret;
   }
   return ret;
 }