Roll pub to the latest version.

This fixes precompiling snapshots when running pub get/upgrade which
broke when the VM removed support for "--preview-dart-2".

Change-Id: I70fe2241f85cf5e8cdf447387df92ea47d398c57
Reviewed-on: https://dart-review.googlesource.com/61440
Commit-Queue: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Kevin Moore <kevmoo@google.com>
Reviewed-by: Kevin Moore <kevmoo@google.com>
diff --git a/DEPS b/DEPS
index ae550dd..c1107d7 100644
--- a/DEPS
+++ b/DEPS
@@ -110,7 +110,7 @@
   "ply_rev": "604b32590ffad5cbb82e4afef1d305512d06ae93",
   "pool_tag": "1.3.4",
   "protobuf_tag": "0.7.1",
-  "pub_rev": "8b9526c915bf21627a20cd0104cb6c2be25a879f",
+  "pub_rev": "c84ffa0265fe746824aa6f382cb6c41abc406d42",
   "pub_semver_tag": "1.4.1",
   "quiver_tag": "5aaa3f58c48608af5b027444d561270b53f15dbf",
   "resource_rev":"af5a5bf65511943398146cf146e466e5f0b95cb9",