Remove the concept of document.documentElement

sky/tests/lowlevel/camel-case.sky

 <CamelCase>
 <script>
 import "../resources/third_party/unittest/unittest.dart";
 import "../resources/unit.dart";
 
 import "dart:sky";
 
 void main () {
   initUnit();
 
   test('should have various casing', () {
-    expect(document.documentElement.tagName, equals('CamelCase'));
+    expect(document.firstElementChild.tagName, equals('CamelCase'));
 
     var element = document.createElement('CamelCase');
     expect(element.tagName, equals('CamelCase'));
   });
 }
 </script>
 </CamelCase>