Revert "Update to the latest package:collection"

This reverts commit e1393eb.

Reason for revert: Temporarily reverting to decide how to
handle breakage in google3.

The new extension methods cause conflicts in some targets.

Original change's description:
> Update to the latest package:collection
>
> Change-Id: I08caf8a015f100d8a07242e8eb25123effcdcf12
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/200929
> Auto-Submit: Nate Bosch <[email protected]>
> Commit-Queue: Nate Bosch <[email protected]>
> Commit-Queue: Jake Macdonald <[email protected]>
> Reviewed-by: Jake Macdonald <[email protected]>

[email protected],[email protected]

Change-Id: I621c49907dbc897b569db805afb76857bab8405e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/201040
Reviewed-by: Nate Bosch <[email protected]>

Author: natebosch

DEPS

@@ -85,7 +85,7 @@ vars = {
   "chrome_rev" : "19997",
   "cli_util_rev" : "8c504de5deb08fe32ecf51f9662bb37d8c708e57",
   "clock_rev" : "a494269254ba978e7ef8f192c5f7fec3fc05b9d3",
-  "collection_rev": "ae0725c97130756d59d8b9b87c0bb0e3e9f3eada",
+  "collection_rev": "9967dcd3d7645db6de48d5abfab3018bb0c84236",
   "convert_rev": "a60156c6efd653657c8926b5788219ed609917d7",
   "crypto_rev": "1c8ccc07b83b100216dc6dede767371043385648",
   "csslib_rev": "e411d862fd8cc50415c1badf2632e017373b3f47",