Fixes two type warnings in test_runner.dart

tools/testing/dart/test_runner.dart

Index: tools/testing/dart/test_runner.dart
diff --git a/tools/testing/dart/test_runner.dart b/tools/testing/dart/test_runner.dart
index 34d193dda321534ecf0e4ff69eb8f2070262606e..1146584eb442d2f7334eb65ca3c966d4a5ef040d 100644
--- a/tools/testing/dart/test_runner.dart
+++ b/tools/testing/dart/test_runner.dart
@@ -329,14 +329,16 @@ class AnalysisTestOutputImpl extends TestOutputImpl {
     // Read the returned list of errors and stuff them away.
     for (String line in super.stderr) {
       if (line.length == 0) continue;
+      // An error line has 8 fields that look like:
+      // ERROR|COMPILER|MISSING_SOURCE|file:/tmp/t.dart|15|1|24|Missing source.
       List<String> fields = splitMachineError(line);
       if (fields[0] == 'ERROR') {
-        errors.add(fields);
+        errors.add(fields[7]);
       } else if (fields[0] == 'WARNING') {
         // We only care about testing Static type warnings
         // ignore all others
         if (fields[1] == 'STATIC_TYPE') {
-          staticWarnings.add(fields);
+          staticWarnings.add(fields[7]);
         }
       }
       // OK to Skip error output that doesn't match the machine format