Make element.style["color"] work in Sky

sky/tests/lowlevel/bug-438036.sky

 <sky>
 <style>
   div { width: 100px; height: 100px; background-color: red; }
   div.wide { width: 200px; }
   div#high { height: 200px; }
 </style>
 <div></div>
 <div></div>
 <script>
 import "../resources/third_party/unittest/unittest.dart";
 import "../resources/unit.dart";
 
 import "dart:sky";
 
 void main() {
   initUnit();
 
   test("should grow width to 200px", () {
     var target = document.querySelector('div');
 
     target.classList.add("wide");
-    expect(window.getComputedStyle(target).getPropertyValue("width"),
+    expect(window.getComputedStyle(target)["width"],
            equals("200px"));
   });
 
   test("should grow height to 200px", () {
     var target = document.querySelectorAll('div')[1];
 
     target.setAttribute("id", "high");
-    expect(window.getComputedStyle(target).getPropertyValue("height"),
+    expect(window.getComputedStyle(target)["height"],
            equals("200px"));
   });
 }
 </script>
 </sky>