Add property to CssStyleDeclaration to detect if a CSS value is available.

tests/html/cssstyledeclaration_test.dart

Index: tests/html/cssstyledeclaration_test.dart
diff --git a/tests/html/cssstyledeclaration_test.dart b/tests/html/cssstyledeclaration_test.dart
index f506c5c9840e3be515f83ac26b35eb0468db0631..b146085a835939d848b3f2017af25b50bce50a1b 100644
--- a/tests/html/cssstyledeclaration_test.dart
+++ b/tests/html/cssstyledeclaration_test.dart
@@ -150,4 +150,11 @@ main() {
     elements = document.queryAll('li');
     expect(elements.style.borderLeftWidth, equals('10px'));
   });
+
+  test('supports property', () {
+    expect(document.body.style.supportsProperty('bogus-property'), false);
+    expect(document.body.style.supportsProperty('background'), true);
+    expect(document.body.style.supportsProperty('borderBottomWidth'), true);
+    expect(document.body.style.supportsProperty('animation'), true);
+  });
 }