Make client tests return non-zero exit code on failure.

client/testing/unittest/unittest_vm.dart

Index: client/testing/unittest/unittest_vm.dart
diff --git a/client/testing/unittest/unittest_vm.dart b/client/testing/unittest/unittest_vm.dart
index 55fbd440efd6676be69e769169a70079e418aee1..9b185380255b51270c0d65845e2fb226e7e4e39f 100644
--- a/client/testing/unittest/unittest_vm.dart
+++ b/client/testing/unittest/unittest_vm.dart
@@ -36,11 +36,18 @@ _platformCompleteTests(int testsPassed, int testsFailed, int testsErrors) {
   // Show the summary.
   print('');
 
+  var success = false;
   if (testsPassed == 0 && testsFailed == 0 && testsErrors == 0) {
     print('No tests found.');
+    // This is considered a failure too: if this happens you probably have a
+    // bug in your unit tests.
   } else if (testsFailed == 0 && testsErrors == 0) {
     print('All $testsPassed tests passed.');
+    success = true;
   } else {
     print('$testsPassed PASSED, $testsFailed FAILED, $testsErrors ERRORS');
   }
+
+  // A non-zero exit code is used by the test infrastructure to detect failure.
+  if (!success) exit(1);
 }