Improve return type of markUnmodifiableList to JSArray instead of List.

sdk/lib/_internal/compiler/js_lib/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 _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
@@ skipping 65 matching lines @@
     JS('void', r'#.fixed$length = Array', list);
     return JS('JSFixedArray', '#', list);
   }
 
   static List markUnmodifiableList(List list) {
     // Functions are stored in the hidden class and not as properties in
     // the object. We never actually look at the value, but only want
     // to know if the property exists.
     JS('void', r'#.fixed$length = Array', list);
     JS('void', r'#.immutable$list = Array', list);
-    return JS('List', '#', list);
+    // TODO(23309): Make it detectable that the list has fixed length.
+    return JS('JSArray', '#', list);
   }
 
   checkMutable(reason) {
     if (this is !JSMutableArray) {
       throw new UnsupportedError(reason);
     }
   }
 
   checkGrowable(reason) {
     if (this is !JSExtendableArray) {
@@ skipping 542 matching lines @@
 
     if (_index >= length) {
       _current = null;
       return false;
     }
     _current = _iterable[_index];
     _index++;
     return true;
   }
 }