Add Iterable for key value pair to HashMap

[DartBot]

This issue was originally filed by @paulevans

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HashMap has:
  external Iterable<K> get keys;
  external Iterable<V> get values;

But does not expose an Iterable for the pairs. It only exposes that via:

external void forEach(void action(K key, V value));

It would be useful to use functions such as fold that can be found on Iterable on the key value pair collection.

[iposva-google]

Added Area-Library, Triaged labels.

[DartBot]

This comment was originally written by @seaneagan

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dupe of issue #7088

[lrhn]

Added Duplicate label.
Marked as being merged into #7088.