[code-workspace] ignore dart2js swarm demo

This code is VERY old and makes the modern analyzer very sad

Change-Id: I178bf208ab73b3e66bec1ff630e3556adfc13fd6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/501761
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Kevin Moore <kevmoo@google.com>
Auto-Submit: Kevin Moore <kevmoo@google.com>

sdk.code-workspace

@@ -72,6 +72,9 @@
       // since it's in .gitignore, but it doesn't seem to.
       "xcodebuild",
       "out",
+
+      // A VERY old benchmark that has not been updated for the latest SDK
+      "pkg/compiler/test/codesize/swarm",
     ],
   },
 }