pkg/unittest: fixed deprecations in alt configs

pkg/unittest/lib/compact_vm_config.dart

Index: pkg/unittest/lib/compact_vm_config.dart
diff --git a/pkg/unittest/lib/compact_vm_config.dart b/pkg/unittest/lib/compact_vm_config.dart
index ae8ec971b17d0378c7f9922fbc7c3d964af3682a..080824ae5dfb40b64ef845255ff9707c528257a7 100644
--- a/pkg/unittest/lib/compact_vm_config.dart
+++ b/pkg/unittest/lib/compact_vm_config.dart
@@ -88,7 +88,7 @@ class CompactVMConfiguration extends VMConfiguration {
       [String color = _NONE]) {
     var duration = (new DateTime.now()).difference(startTime);
     var buffer = new StringBuffer();
-    // \r moves back to the beginnig of the current line.
+    // \r moves back to the beginning of the current line.
     buffer.write('\r${_timeString(duration)} ');
     buffer.write(_GREEN);
     buffer.write('+');
@@ -121,7 +121,7 @@ class CompactVMConfiguration extends VMConfiguration {
         _lastLength--;
       }
     }
-    stdout.addString(buffer.toString());
+    stdout.write(buffer.toString());
   }
 
   String _padTime(int time) =>