Ensure that hashCode and runtimeType work on null.

lib/core/map.dart

Index: lib/core/map.dart
diff --git a/lib/core/map.dart b/lib/core/map.dart
index 98519c949a7e93b4ab888f8ba5f895d2d425bf3d..cc3a01ed03dca482218e96ebb3cf26d69b041e7c 100644
--- a/lib/core/map.dart
+++ b/lib/core/map.dart
@@ -4,7 +4,7 @@
 
 /**
  * A [Map] is an associative container, mapping a key to a value.
- * Null values are supported.
+ * Null values are supported, but null keys are not.

[ahe, 2012/09/27 09:38:51]

Why have this limitation?

[Lasse Reichstein Nielsen, 2012/09/27 09:55:37]

It was always there.
The code as written assumed null wouldn't be Hashable, so it used null as an
internal marker.
The implementation should probably be changed, but that's outside the scope of
this CL.

[ahe, 2012/09/27 10:17:21]

You're changing the specification in this CL. I think that's outside the scope
of a dart2js change.

If the implementation is wrong, how about filing a bug?

[Lasse Reichstein Nielsen, 2012/09/27 10:35:53]

I don't think I'm changing the specification. Saying "Null values are supported"
but not mentioning keys at least suggests that null keys are not. 
The implementation never supported null keys.

I'm changing something else, but keeping the behavior of Map exactly as before,
and making the implicit contract explicit.
If there was a bug, it was in the documentation being vague, which this change
fixes.

Now, for a later change of the Library, we *may* want to allow null keys. I'm
not actually sure I like that, but it'll be consistent with other containers
that allow null (like List). When that happens, the Map implementation will need
to change. And if we don't want null keys, I want to throw an ArgumentError
instead of a NullPointerException, so the implementation will have to change
anyway. That will be a change to HashMap.

[ahe, 2012/09/27 10:45:33]

This all sounds like something that needs to be recorded in a bug report, not a
review comment.

  */
 interface Map<K, V> default HashMapImplementation<K extends Hashable, V> {
   /**