remove redundant type check in DDC's SDK implementation

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

Index: pkg/dev_compiler/tool/input_sdk/private/js_array.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/js_array.dart b/pkg/dev_compiler/tool/input_sdk/private/js_array.dart
index 2d4d0b481e9f69e79744071c09cf45315f035623..2a3cfe7ee29c999f38bbc7e81d7c56a3c7ce9d13 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/js_array.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/js_array.dart
@@ -21,7 +21,7 @@ class JSArray<E> implements List<E>, JSIndexable<E> {
   factory JSArray.of(allocation) {
     // TODO(sra): Move this to core.List for better readability.
     // Capture the parameterized ES6 'JSArray' class.
-    return dart.setType(allocation, JS('', 'JSArray'));
+    return JS('-dynamic', '#', dart.setType(allocation, JS('', 'JSArray')));
   }
 
   // TODO(jmesserly): consider a fixed array subclass instead.