Fix the return type of String.split in DDC to be correct

pkg/dev_compiler/tool/input_sdk/private/js_string.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 [String]. 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 79 matching lines @@
   String replaceFirstMapped(
       Pattern from, @nullCheck String replace(Match match),
       [@nullCheck int startIndex = 0]) {
     RangeError.checkValueInInterval(startIndex, 0, this.length, "startIndex");
     return stringReplaceFirstMappedUnchecked(this, from, replace, startIndex);
   }
 
   @notNull
   List<String> split(@nullCheck Pattern pattern) {
     if (pattern is String) {
-      return JS('JSExtendableArray', r'#.split(#)', this, pattern);
+      return new JSArray.of(JS('', r'#.split(#)', this, pattern));
     } else if (pattern is JSSyntaxRegExp && regExpCaptureCount(pattern) == 0) {
       var re = regExpGetNative(pattern);
-      return JS('JSExtendableArray', r'#.split(#)', this, re);
+      return new JSArray.of(JS('', r'#.split(#)', this, re));
     } else {
       return _defaultSplit(pattern);
     }
   }
 
   @notNull
   String replaceRange(
       @nullCheck int start, int end, @nullCheck String replacement) {
     end = RangeError.checkValidRange(start, end, this.length);
     return stringReplaceRangeUnchecked(this, start, end, replacement);
@@ skipping 385 matching lines @@
   int get length => JS('int', r'#.length', this);
 
   @notNull
   String operator [](@nullCheck int index) {
     if (index >= JS('int', '#.length', this) || index < 0) {
       throw diagnoseIndexError(this, index);
     }
     return JS('String', '#[#]', this, index);
   }
 }