[infra] Add vm-kernel-legacy builder to the test matrix

https://github.com/dart-lang/sdk/issues/32499

TBR=whesse@google.com

Change-Id: Ia9829fc106162b161e9e31bc918feebc691b3621
Reviewed-on: https://dart-review.googlesource.com/46742
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
diff --git a/tools/bots/test_matrix.json b/tools/bots/test_matrix.json
index a1580fe..14c06a3 100644
--- a/tools/bots/test_matrix.json
+++ b/tools/bots/test_matrix.json
@@ -211,6 +211,27 @@
     },
     {
       "builders": [
+        "vm-kernel-legacy-linux-release-x64"
+      ],
+      "meta": {
+        "description": "Tests the vm with dartk in legacy mode."
+      },
+      "steps": [
+        {
+          "name": "build dart",
+          "script": "tools/build.py",
+          "arguments": ["runtime_kernel"]
+        },
+        {
+          "name": "vm legacy tests",
+          "arguments": ["--compiler=dartk"],
+          "fileset": "vm-kernel",
+          "shards": 10
+        }
+      ]
+    },
+    {
+      "builders": [
         "vm-kernel-precomp-linux-release-simarm",
         "vm-kernel-precomp-linux-release-simarm64",
         "vm-kernel-precomp-win-release-x64"