Adding supported checks and flags to window.performance.

tests/html/performance_api_test.dart

 library PerformanceApiTest;
 import '../../pkg/unittest/lib/unittest.dart';
-import '../../pkg/unittest/lib/html_config.dart';
+import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:html';
 
 main() {
-  useHtmlConfiguration();
-  test('PerformanceApi', () {
-    // Check that code below will not throw exceptions.
-    var requestStart = window.performance.timing.requestStart;
-    var responseStart = window.performance.timing.responseStart;
-    var responseEnd = window.performance.timing.responseEnd;
-    
-    var loading = window.performance.timing.domLoading;
-    var loadedStart = window.performance.timing.domContentLoadedEventStart;
-    var loadedEnd = window.performance.timing.domContentLoadedEventEnd;
-    var complete = window.performance.timing.domComplete;
+  useHtmlIndividualConfiguration();
 
-    var loadEventStart = window.performance.timing.loadEventStart;
+  group('supported', () {
+    test('supported', () {
+      expect(Performance.supported, true);
+    });
+  });
+
+  group('performance', () {
+    test('PerformanceApi', () {
+      // Check that code below will not throw exceptions if supported.
+      var expectation = Performance.supported ? returnsNormally : throws;
+      expect(() {
+        var requestStart = window.performance.timing.requestStart;
+        var responseStart = window.performance.timing.responseStart;
+        var responseEnd = window.performance.timing.responseEnd;
+
+        var loading = window.performance.timing.domLoading;
+        var loadedStart = window.performance.timing.domContentLoadedEventStart;
+        var loadedEnd = window.performance.timing.domContentLoadedEventEnd;
+        var complete = window.performance.timing.domComplete;
+
+        var loadEventStart = window.performance.timing.loadEventStart;
+      }, expectation);
+    });
   });
 }