Expand the version constraint to allow using csslib 0.13.x.

I've pushed out new versions of csslib that are strong mode clean. They
technically have some breaking changes, so I bumped the version past
0.12.0, but in practice html works with them just fine. This allows
that.

R=jmesserly@google.com

Review URL: https://codereview.chromium.org//1907013002 .
diff --git a/CHANGELOG.md b/CHANGELOG.md
index f4861d1..4dbea39 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,9 @@
 This file contains highlights of what changes on each version of the html
 package.
 
+#### Pub version 0.12.2+2
+  * Support `csslib` versions `0.13.x`.
+  
 #### Pub version 0.12.2+1
   * Exclude `.packages` file from the published package.
 
diff --git a/pubspec.yaml b/pubspec.yaml
index 64c8ea6..697d1c8 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,12 +1,12 @@
 name: html
-version: 0.12.2+1
+version: 0.12.2+2
 author: Dart Team <misc@dartlang.org>
 description: A library for working with HTML documents. Previously known as html5lib.
 homepage: https://github.com/dart-lang/html
 environment:
   sdk: '>=1.2.0 <2.0.0'
 dependencies:
-  csslib: '>=0.10.0 <0.13.0'
+  csslib: '>=0.10.0 <0.14.0'
   source_span: '>=1.0.0 <2.0.0'
   utf: '>=0.9.0 <0.10.0'
 dev_dependencies: