Fix handling of negative exit codes in test.dart on Android.

tools/testing/dart/android.dart

Index: tools/testing/dart/android.dart
diff --git a/tools/testing/dart/android.dart b/tools/testing/dart/android.dart
index 9a3de01627bf7845879782e1351c568149898742..bf83128d37353670ef1292686363cf47953c822d 100644
--- a/tools/testing/dart/android.dart
+++ b/tools/testing/dart/android.dart
@@ -333,6 +333,7 @@ class AdbDevice {
       if (index >= 0) {
         exitCode = int.parse(
             lines.last.substring(index + MARKER.length).trim());
+        exitCode = exitCode.toSigned(8);
       } else {
         // In case of timeouts, for example, we won't get the exitcode marker.
         assert(result.exitCode != 0);