Bump pub version

Changes:
```
> git log --format="%C(auto) %h %s" 15a46117da29cc572fba620241c83a2117cdae09..a817863ee93241ff36fce6856c6d12fd8fde0907
 a817863e Use pool for file reading (#3156)
 59237e29 Don't ask packageLister for availabe versions (#3151)
 f2f86c01 Fix analyze errors (#3148)
 39c9779c Environment credentials support (#3115)
 f0020823 Improved handling for authentication errors (#3120)
 1bb9f923 Migrate ignore.dart to null-safety (#3142)

```

Change-Id: I01d6637e3de8e523596394383393f5eda1a728c9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/214804
Reviewed-by: Jonas Jensen <[email protected]>
Commit-Queue: Jonas Jensen <[email protected]>

Author: szakarias

DEPS

@@ -139,7 +139,7 @@ vars = {
   "pool_rev": "7abe634002a1ba8a0928eded086062f1307ccfae",
   "process_rev": "56ece43b53b64c63ae51ec184b76bd5360c28d0b",
   "protobuf_rev": "c1eb6cb51af39ccbaa1a8e19349546586a5c8e31",
-  "pub_rev": "15a46117da29cc572fba620241c83a2117cdae09",
+  "pub_rev": "a817863ee93241ff36fce6856c6d12fd8fde0907",
   "pub_semver_rev": "a43ad72fb6b7869607581b5fedcb186d1e74276a",
   "resource_rev": "6b79867d0becf5395e5819a75720963b8298e9a7",
   "root_certificates_rev": "692f6d6488af68e0121317a9c2c9eb393eb0ee50",