Revert "Update fixnum to the migrated version and add it to the allowed experiments file"

This reverts commit cb1105a.

Reason for revert: Broke flutter-engine-linux bot https://ci.chromium.org/p/dart/builders/ci.sandbox/flutter-engine-linux/11048?

Original change's description:
> Update fixnum to the migrated version and add it to the allowed experiments file
> 
> This package is used in benchmarking and we need a migrated version in order
> to run benchmarks with null safety enabled.
> 
> Change-Id: I916331a03006d6b5d3f0d0bb590012376e410c24
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150170
> Commit-Queue: Jake Macdonald <[email protected]>
> Reviewed-by: Stephen Adams <[email protected]>

[email protected],[email protected],[email protected]

Change-Id: I4536f395a877c245140d53f7a9463f2ed156d012
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151926
Reviewed-by: Jake Macdonald <[email protected]>
Auto-Submit: Jake Macdonald <[email protected]>
Commit-Queue: Jake Macdonald <[email protected]>

Author: jakemac53

.dart_tool/package_config.json

@@ -11,7 +11,7 @@
     "constraint, update this by running tools/generate_package_config.dart."
   ],
   "configVersion": 2,
-  "generated": "2020-06-22T07:14:12.580733",
+  "generated": "2020-06-17T09:39:51.245406",
   "generator": "tools/generate_package_config.dart",
   "packages": [
     {
@@ -225,7 +225,7 @@
       "name": "fixnum",
       "rootUri": "../third_party/pkg/fixnum",
       "packageUri": "lib/",
-      "languageVersion": "2.9"
+      "languageVersion": "2.1"
     },
     {
       "name": "front_end",
@@ -700,4 +700,4 @@
       "languageVersion": "2.4"
     }
   ]
-}
+}

DEPS

@@ -98,7 +98,7 @@ vars = {
 
   "dartdoc_tag" : "v0.32.1",
   "ffi_rev": "454ab0f9ea6bd06942a983238d8a6818b1357edb",
-  "fixnum_rev": "9b38f49f6679654d66a363e69e48173cca07e882",
+  "fixnum_rev": "13da5123989a5d91cb59e76793c3923dd8d06e90",
   "glob_rev": "e9f4e6b7ae8abe5071461cf8f47191bb19cf7ef6",
   "html_rev": "083a36cd801a4b787ba156b7c6e4c8b2e2daed4a",
   "http_io_rev": "2fa188caf7937e313026557713f7feffedd4978b",