Add workaround for bug adding unicode strings to test reports. (#145607)

When print is used inside tests its content is added to the test report as a string of unicode \u0000 making the test reporting parser fail. This PR cleans removes non json content every line of the report.

Bug: https://github.com/flutter/flutter/issues/145553

dev/bots/tool_subsharding.dart

@@ -49,7 +49,14 @@ class TestFileReporterResults {
     final List<String> errors = <String>[];
 
     for (final String metric in metrics.readAsLinesSync()) {
-      final Map<String, Object?> entry = json.decode(metric) as Map<String, Object?>;
+      /// Using print within a test adds the printed content to the json file report
+      /// as \u0000 making the file parsing step fail. The content of the json file
+      /// is expected to be a json dictionary per line and the following line removes
+      /// all the additional content at the beginning of the line until it finds the
+      /// first opening curly bracket.
+      // TODO(godofredoc): remove when https://github.com/flutter/flutter/issues/145553 is fixed.
+      final String sanitizedMetric = metric.replaceAll(RegExp(r'$.*{'), '{');
+      final Map<String, Object?> entry = json.decode(sanitizedMetric) as Map<String, Object?>;
       if (entry.containsKey('suite')) {
         final Map<String, Object?> suite = entry['suite']! as Map<String, Object?>;
         addTestSpec(suite, entry['time']! as int, testSpecs);