Create 'unified' version of Date, with a dart2js implementation.

lib/compiler/implementation/lib/coreimpl.dartp

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Patch file for dart:core's Date class.
+
+patch class DateImplementation {
+  patch DateImplementation(int years,
+                           [int month = 1,
+                            int day = 1,
+                            int hour = 0,
+                            int minute = 0,
+                            int second = 0,
+                            int millisecond = 0,
+                            bool isUtc = false])
+      : this.isUtc = checkNull(isUtc),
+        millisecondsSinceEpoch = Primitives.valueFromDecomposedDate(
+            years, month, day, hour, minute, second, millisecond, isUtc) {
+    Primitives.lazyAsJsDate(this);
+  }
+
+  patch DateImplementation.now()
+      : isUtc = false,
+        millisecondsSinceEpoch = Primitives.dateNow() {
+    Primitives.lazyAsJsDate(this);
+  }
+
+  patch String get timeZoneName() {
+    if (isUtc) return "UTC";
+    return Primitives.getTimeZoneName(this);
+  }
+
+  patch Duration get timeZoneOffset() {
+    if (isUtc) return new Duration(0);
+    return new Duration(minutes: Primitives.getTimeZoneOffsetInMinutes(this));
+  }
+
+  patch int get year() => Primitives.getYear(this);
+
+  patch int get month() => Primitives.getMonth(this);
+
+  patch int get day() => Primitives.getDay(this);
+
+  patch int get hour() => Primitives.getHours(this);
+
+  patch int get minute() => Primitives.getMinutes(this);
+
+  patch int get second() => Primitives.getSeconds(this);
+
+  patch int get millisecond() => Primitives.getMilliseconds(this);
+
+  patch int get weekday() {
+    // Adjust by one because JS weeks start on Sunday.
+    var day = Primitives.getWeekday(this);

[floitsch, 2012/08/01 09:05:01]

We should be consistent: either we let the Primitives deal with the different
offsets, or we deal with them here.
In particular: Primitives.getMonth(this) needs to map from JS 0-based to Dart's
1-based months. The mapping from JS' weekday to Dart's weekday should happen at
the same level.

[Anders Johnsen, 2012/08/01 09:15:38]

Done.

+    return (day + 6) % 7 + Date.MON;
+  }
+}