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

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

Index: pkg/dev_compiler/tool/input_sdk/private/js_string.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/js_string.dart b/pkg/dev_compiler/tool/input_sdk/private/js_string.dart
index 08a9edb7bb5260cb900dc523232f559223a00166..3ca371308299a21a342625e75460ceed53cc0bb9 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/js_string.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/js_string.dart
@@ -97,10 +97,10 @@ class JSString extends Interceptor implements String, JSIndexable<String> {
   @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);
     }