Remove deprecated StringBuffer.add, addAll and addCharCode.

pkg/yaml/lib/parser.dart

Index: pkg/yaml/lib/parser.dart
diff --git a/pkg/yaml/lib/parser.dart b/pkg/yaml/lib/parser.dart
index 386bb591240736300cbf0f8d1218dd1bcc7b4c53..c84f3741d94ae3c92edb6872552b336b7ca9ef3e 100644
--- a/pkg/yaml/lib/parser.dart
+++ b/pkg/yaml/lib/parser.dart
@@ -347,13 +347,13 @@ class _Parser {
     capturingAs = false;
     if (!truth(res)) return res;
 
-    capturedString.add(transformation(s.substring(captureStart, pos)));
+    capturedString.write(transformation(s.substring(captureStart, pos)));
     captureStart = pos;
     return res;
   }
 
   void flushCapture() {
-    capturedString.add(s.substring(captureStart, pos));
+    capturedString.write(s.substring(captureStart, pos));
     captureStart = pos;
   }