Remove deprecated StringBuffer.add, addAll and addCharCode.

sdk/lib/_internal/compiler/implementation/constants.dart

Index: sdk/lib/_internal/compiler/implementation/constants.dart
diff --git a/sdk/lib/_internal/compiler/implementation/constants.dart b/sdk/lib/_internal/compiler/implementation/constants.dart
index b9388df303c7349a2c1c856afc5dc382a2dfb212..eae62eadb32ddf5ac981f8b937b4ec693f5e7a6d 100644
--- a/sdk/lib/_internal/compiler/implementation/constants.dart
+++ b/sdk/lib/_internal/compiler/implementation/constants.dart
@@ -128,7 +128,7 @@ class NullConstant extends PrimitiveConstant {
   }
 
   void _writeJsCode(CodeBuffer buffer, ConstantHandler handler) {
-    buffer.add(JsNull);
+    buffer.write(JsNull);
   }
 
   // The magic constant has no meaning. It is just a random value.