Improve return type of markUnmodifiableList to JSArray instead of List.

CHANGELOG.md

 ## 1.11.0
 
 ### Core library changes
 
+ * Add `unmodifiable` constructor to `List` class - 
+   [r45334](https://code.google.com/p/dart/source/detail?r=45334),
 * Update experimental Isolate API:
   - Make priorty parameters of `Isolate.ping` and `Isolate.kill` methods
     a named parameter.
   - Remove the `Isolate.AS_EVENT` priority.
   - Add extra `response` parameter to `Isolate.ping` and
     `Isolate.addOnExitListener`.
   [r45092](https://code.google.com/p/dart/source/detail?r=45092),
 
 ## 1.10.0
 
 ### Core library changes
 
-* Fix behavior of `HtmlEscape` in `dart:convert`. No longer escape
+* Fix behavior of `HtmlEscape` in `dart:convert`. It no longer escapes
   no-break space (U+00A0) anywhere or forward slash ('/', U+002F) in element
   context. Slash is still escaped in the "unknown" context.
   [r45003](https://code.google.com/p/dart/source/detail?r=45003),
   [r45153](https://code.google.com/p/dart/source/detail?r=45153),
   [r45189](https://code.google.com/p/dart/source/detail?r=45189)
 
 * dart:html: CssClassSet method arguments must now be 'tokens', i.e. non-empty
   strings with no white-space characters. The implementation was incorrect for
   class names containing spaces. The fix is to forbid spaces and provide a
   faster implementation.
@@ skipping 232 matching lines @@
     they will keep the Dart process alive until they time out. This fixes the
     handling of persistent connections. Previously, the client would shut down
     immediately after a request.
 
   * **Breaking change:** `HttpServer` no longer compresses all traffic by
     default. The new `autoCompress` property can be set to `true` to re-enable
     compression.
 
 * `dart:isolate`: `Isolate.spawnUri` added the optional `packageRoot` argument,
   which controls how it resolves `package:` URIs.