Fixed 3 final missing casts in dart:js

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

Index: tool/input_sdk/lib/js/dart2js/js_dart2js.dart
diff --git a/tool/input_sdk/lib/js/dart2js/js_dart2js.dart b/tool/input_sdk/lib/js/dart2js/js_dart2js.dart
index 6fde3c644e2e76f93b7fc0667906e5bffe6f9142..4169bed09284d98d15d9616f6f3033bfa6dd444d 100644
--- a/tool/input_sdk/lib/js/dart2js/js_dart2js.dart
+++ b/tool/input_sdk/lib/js/dart2js/js_dart2js.dart
@@ -357,7 +357,7 @@ class JsArray<E> extends JsObject with ListMixin<E> {
     if (index is num && index == index.toInt()) {
       _checkIndex(index);
     }
-    return super[index];
+    return super[index] as E;
   }
 
   void operator []=(Object index, E value) {
@@ -403,12 +403,12 @@ class JsArray<E> extends JsObject with ListMixin<E> {
 
   E removeAt(int index) {
     _checkIndex(index);
-    return callMethod('splice', [index, 1])[0];
+    return callMethod('splice', [index, 1])[0] as E;
   }
 
   E removeLast() {
     if (length == 0) throw new RangeError(-1);
-    return callMethod('pop');
+    return callMethod('pop') as E;
   }
 
   void removeRange(int start, int end) {