Optimize String.fromCharCode.

sdk/lib/_internal/lib/core_patch.dart

Index: sdk/lib/_internal/lib/core_patch.dart
diff --git a/sdk/lib/_internal/lib/core_patch.dart b/sdk/lib/_internal/lib/core_patch.dart
index 30aa9264d8b96b8f071145c78eae3f3040973a62..926b7795ec79ceb602deeae6e8ccdf51cbed3f7d 100644
--- a/sdk/lib/_internal/lib/core_patch.dart
+++ b/sdk/lib/_internal/lib/core_patch.dart
@@ -219,6 +219,10 @@ patch class String {
     return Primitives.stringFromCharCodes(charCodes);
   }
 
+  patch factory String.fromCharCode(int charCode) {
+    return Primitives.stringFromCharCode(charCode);
+  }
+
   patch factory String.fromEnvironment(String name, {String defaultValue}) {
     throw new UnsupportedError(
         'String.fromEnvironment can only be used as a const constructor');