Ensure that hashCode and runtimeType work on null.

lib/core/map.dart

 // Copyright (c) 2011, 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.
 
 /**
  * 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> {
   /**
    * Creates a map with the default implementation.
    */
   Map();
 
   /**
    * Creates a [Map] that contains all key value pairs of [other].
    */
@@ skipping 96 matching lines @@
   /**
    * Creates a map with the default implementation.
    */
   LinkedHashMap();
 
   /**
    * Creates a [LinkedHashMap] that contains all key value pairs of [other].
    */
   LinkedHashMap.from(Map<K, V> other);
 }