HttpHeaders should be a Map, or at least support enumeration of headers

[jmesserly]

From Eric J Smith:
Unable to iterate headers. I should be able to iterate the header names and values and get count. Why doesn't HttpHeaders derive from Map?

https://groups.google.com/a/dartlang.org/forum/#!topic/misc/6GBKm5fOrqA

[jmesserly]

BTW, I noticed that _HttpHeaders actually does have a map internally:

class _HttpHeaders implements HttpHeaders {
  _HttpHeaders() : _headers = new Map<String, List<String>>();
 ...

[jmesserly]

Added Duplicate label.
Marked as being merged into #2045.

[kevmoo]

Removed Area-IO label.
Added Area-Library, Library-IO labels.