Fix more errors in DDC SDK.

pkg/dev_compiler/tool/input_sdk/lib/js/dart2js/js_dart2js.dart

Index: pkg/dev_compiler/tool/input_sdk/lib/js/dart2js/js_dart2js.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/lib/js/dart2js/js_dart2js.dart b/pkg/dev_compiler/tool/input_sdk/lib/js/dart2js/js_dart2js.dart
index 9e4133148615846b282ece787f044e4658443e8f..a4f28fe77d1cd81a3f85940ed058ac8eba81a879 100644
--- a/pkg/dev_compiler/tool/input_sdk/lib/js/dart2js/js_dart2js.dart
+++ b/pkg/dev_compiler/tool/input_sdk/lib/js/dart2js/js_dart2js.dart
@@ -204,7 +204,7 @@ class JsObject {
    *
    * 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");
     }
@@ -367,7 +367,7 @@ class JsArray<E> extends JsObject with ListMixin<E> {
     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()) {