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

lib/compiler/implementation/compiler.dart

Index: lib/compiler/implementation/compiler.dart
diff --git a/lib/compiler/implementation/compiler.dart b/lib/compiler/implementation/compiler.dart
index abc40e3844ecb79c6f81597dd1aa5ef21c544333..a9d73b53b26bb5d748839cf2d1b0d87c0efc7162 100644
--- a/lib/compiler/implementation/compiler.dart
+++ b/lib/compiler/implementation/compiler.dart
@@ -556,8 +556,10 @@ class Compiler implements DiagnosticListener {
     while (!patches.isEmpty()) {
       Element patchElement = patches.head;
       Element originalElement = lookup(patchElement.name);
-      // Getters and setters are kept inside a synthetic field.
-      if (patchElement.kind === ElementKind.ABSTRACT_FIELD) {
+      if (patchElement.isAccessor()) {
+        // Skip accessors, since we handle them in the abstract field case.

[Mads Ager (google), 2012/08/01 08:08:31]

Does this mean that there is both an accessor and an abstract field patch
element in the list for a getter or setter?

[Anders Johnsen, 2012/08/01 08:35:50]

Yep. It's a invariant on the AST. I've updated the comment to be more clear
about this.

+      } else if (patchElement.kind === ElementKind.ABSTRACT_FIELD) {
+        // Getters and setters are kept inside a synthetic field.
         if (originalElement !== null &&
             originalElement.kind !== ElementKind.ABSTRACT_FIELD) {
           internalError("Cannot patch non-getter/setter with getter/setter",