[front_end] Remove dead code regarding null values in collections of non-null values

These will soon be reported by the analyzer for
https://github.com/dart-lang/sdk/issues/57101.

Change-Id: I7fe2a9dca107c8c69b5dff0f527fa501c41a2c74
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/489503
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

pkg/_fe_analyzer_shared/lib/src/testing/id_testing.dart

@@ -1199,11 +1199,6 @@ bool skipForConfig(
       print("Skip: ${testName} for config '${configMarker}'");
       return true;
     }
-    skipList = skipMap[null];
-    if (skipList != null && skipList.contains(testName)) {
-      print("Skip: ${testName} for config '${configMarker}'");
-      return true;
-    }
   }
   return false;
 }

pkg/front_end/test/incremental_utils.dart

@@ -74,7 +74,6 @@ Future<void> throwOnInsufficientUriToSource(
   component.accept(uriFinder);
   Set<Uri> uris = uriFinder.seenUris.toSet();
   uris.removeAll(component.uriToSource.keys);
-  uris.remove(null);
   if (uris.length != 0) {
     throw "Expected 0 uris with no source, but found ${uris.length} ($uris)";
   }