Corrected spelling mistake from "happend" to "happen" throughout multiple files.

tests/standalone/io/file_stream_test.dart

Index: tests/standalone/io/file_stream_test.dart
diff --git a/tests/standalone/io/file_stream_test.dart b/tests/standalone/io/file_stream_test.dart
index f7da71789d4d2549daee48d753f8ff036ce8a7dc..e1a9105485fc0256deb26712fb7cc2deb18c4787 100644
--- a/tests/standalone/io/file_stream_test.dart
+++ b/tests/standalone/io/file_stream_test.dart
@@ -29,7 +29,7 @@ void testPauseResumeCancelStream() {
             close();
           }
         }, onDone: () {
-          Expect.fail('the stream was canceled, onDone should not happend');
+          Expect.fail('the stream was canceled, onDone should not happen');
         });
       });
   });