Don't change Date to DateTime in JavaScript code.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index d5811d2bb16441a1276bff69dc5023e2a67b8add..0b7d311019822805c66b376e8479374437955323 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -25604,7 +25604,7 @@ class Window extends EventTarget implements WindowBase native "@*DOMWindow" {
    if ($this.requestAnimationFrame && $this.cancelAnimationFrame) return;
    $this.requestAnimationFrame = function(callback) {
       return window.setTimeout(function() {
-        callback(DateTime.now());
+        callback(Date.now());
       }, 16 /* 16ms ~= 60fps */);
    };
    $this.cancelAnimationFrame = function(id) { clearTimeout(id); }