Make StringBuffer.write easier to inline

sdk/lib/_internal/compiler/js_lib/core_patch.dart

Index: sdk/lib/_internal/compiler/js_lib/core_patch.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/core_patch.dart b/sdk/lib/_internal/compiler/js_lib/core_patch.dart
index 9f5bf1c562b4bdb01baf7416bbc97039de720deb..c0180d8322342b025b898e259fe3fcfa0c2307cb 100644
--- a/sdk/lib/_internal/compiler/js_lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/js_lib/core_patch.dart
@@ -373,13 +373,16 @@ class StringBuffer {
 
   @patch
   void write(Object obj) {
-    String str = obj is String ? obj : "$obj";
-    _contents = Primitives.stringConcatUnchecked(_contents, str);
+    _writeString('$obj');
   }
 
   @patch
   void writeCharCode(int charCode) {
-    write(new String.fromCharCode(charCode));
+    _writeString(new String.fromCharCode(charCode));
+  }
+
+  void _writeString(str) {
+    _contents = Primitives.stringConcatUnchecked(_contents, str);
   }
 
   @patch