Review request for JDK-8147558: Add support for ES6 collections

[Hannes Wallnöfer]

Please review JDK-8147558: Add support for ES6 collections:

http://cr.openjdk.java.net/~hannesw/8147558/

The patch is rather large so here are a few things to point out:

- The Map and Set collections use a custom linked map implementation 
(objects/LinkedMap) instead of using a standard Java collection such as 
java.util.LinkedHashMap. The reason is that the forEach method in those 
objects allows arbitrary modifications to the base collection during 
iteration by the callback function. The way this is done is that 
additions and deletions are reflected in visited nodes except for 
deletion of elements that have already been visited.

See the note in 
http://www.ecma-international.org/ecma-262/6.0/#sec-map.prototype.foreach

LinkedMap is implemented as a linked list that uses a HashMap for 
hashing (or, in other words, a HashMap with linked-list nodes as 
values). While the hash map methods are synchronized, iteration does not 
use any locking. I think it is safe for concurrent use, but please do 
have a look.

- There are now a IS_ACCESSOR_PROPERTY flag and a isAccessorProperty() 
in runtime/Property. This is because ECMA6 defines several native 
properties as accessor properties (as opposed to data properties). In 
ECMA5, accessor properties always meant properties with user-provided 
get and set functions. The difference between data and accessor 
properties of course is that when accessed over the prototype chain 
accessor is called with the object at the start of the prototype chain 
as receiver instead of the  object owning the property. That's just to 
explain these changes. The gist of it is that it is now possible to 
define accessor properties through nasgen and they should behave just 
like user user-defined accessor properties.

- Finally, there are several small issues left in this patch that I know 
of, and probably more that I don't know of. I could have continued 
writing tests for at least another week, but I think it's good enough to 
push it.

Thanks!
Hannes