remove redundant type check in DDC's SDK implementation

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

 // Copyright (c) 2012, 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.
 
 part of dart._interceptors;
 
 /**
  * The interceptor class for [List]. The compiler recognizes this
  * class as an interceptor, and changes references to [:this:] to
  * actually use the receiver of the method, which is generated as an extra
  * argument added to each member.
  */
 @JsPeerInterface(name: 'Array')
 class JSArray<E> implements List<E>, JSIndexable<E> {
   const JSArray();
 
   /**
    * Constructor for adding type parameters to an existing JavaScript
    * Array. Used for creating literal lists.
    */
   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.
   factory JSArray.markFixed(allocation) =>
       new JSArray<E>.of(markFixedList(allocation));
 
   factory JSArray.markGrowable(allocation) = JSArray<E>.of;
 
   static List markFixedList(List list) {
     // Functions are stored in the hidden class and not as properties in
@@ skipping 591 matching lines @@
 
     if (_index >= length) {
       _current = null;
       return false;
     }
     _current = _iterable[_index];
     _index++;
     return true;
   }
 }