Remove deprecated StringBuffer.add, addAll and addCharCode.

runtime/lib/integers.dart

Index: runtime/lib/integers.dart
diff --git a/runtime/lib/integers.dart b/runtime/lib/integers.dart
index 2b3a1ee768fd8204de05ef5eea9c8cb35ae3f9c9..fa3bb8e5c511d1a453227d1b17c1524b0cb17706 100644
--- a/runtime/lib/integers.dart
+++ b/runtime/lib/integers.dart
@@ -197,9 +197,9 @@ class _IntegerImplementation {
       return "0";
     }
     StringBuffer buffer = new StringBuffer();
-    if (isNegative) buffer.add("-");
+    if (isNegative) buffer.write("-");
     for (int i = temp.length - 1; i >= 0; i--) {
-      buffer.add(table[temp[i]]);
+      buffer.write(table[temp[i]]);
     }
     return buffer.toString();
   }