Fix some errors around []= in core types.

pkg/dev_compiler/tool/input_sdk/private/interceptors.dart

Index: pkg/dev_compiler/tool/input_sdk/private/interceptors.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/interceptors.dart b/pkg/dev_compiler/tool/input_sdk/private/interceptors.dart
index 307726b8dcf8ed240860b03ef337a3e7c32e1ca4..daa0e77651a3f56b9a74012c47d6b15c42cc2f6b 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/interceptors.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/interceptors.dart
@@ -44,18 +44,9 @@ class JSBool extends Interceptor implements bool {
  * 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);
-}
-
-/**
- * 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);
+  E operator[](int index);
 }
 
 /**