Fix exit code handling for Android testing.

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 bf83128d37353670ef1292686363cf47953c822d..615d5b0abcbc4bca0773510ab1b7981c47c7e532 100644
--- a/tools/testing/dart/android.dart
+++ b/tools/testing/dart/android.dart
@@ -333,7 +333,10 @@ class AdbDevice {
       if (index >= 0) {
         exitCode = int.parse(
             lines.last.substring(index + MARKER.length).trim());
-        exitCode = exitCode.toSigned(8);
+        if (exitCode > 128 && exitCode <= 128 + 31) {
+          // Return negative exit codes for signals 1..31 (128+N for signal N)
+          exitCode = 128 - exitCode;
+        }
       } else {
         // In case of timeouts, for example, we won't get the exitcode marker.
         assert(result.exitCode != 0);