[infra] Protect against buggy approve_results versions causing an outage.

The bug in question has been fixed, but this makes sure nobody accidentally
uses a buggy version of approve_results and causes builders to fail.

Change-Id: Iec6f87b960878093c7029f042f7d0697eccf5734
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/97514
Reviewed-by: Alexander Thomas <athom@google.com>
diff --git a/tools/approve_results.dart b/tools/approve_results.dart
index ff149c3..afe488f 100755
--- a/tools/approve_results.dart
+++ b/tools/approve_results.dart
@@ -229,6 +229,11 @@
     final approvedResults =
         await loadResultsMapIfExists("${tmpdir.path}/approved_results.json");
 
+    // TODO: Remove 2019-04-08: Discard any invalid pre-approvals made with a
+    // version of approve_results between 065910f0 and a13ac1b4. Pre-approving
+    // a new test could add pre-approvals with null configuration and null name.
+    approvedResults.remove("null:null");
+
     // Construct an object for every test containing its current result,
     // what the last approved result was, and whether it's flaky.
     final tests = <Test>[];