Migrate ObjcProvider to BuiltinProvider from NativeProvider

RELNOTES: None.
PiperOrigin-RevId: 207778688
diff --git a/src/main/java/com/google/devtools/build/lib/packages/BuiltinProvider.java b/src/main/java/com/google/devtools/build/lib/packages/BuiltinProvider.java
index b880c97..039f1ef 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/BuiltinProvider.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/BuiltinProvider.java
@@ -98,4 +98,11 @@
     throw new EvalException(
         loc, String.format("'%s' cannot be constructed from Skylark", getPrintableName()));
   }
+
+  /**
+   * Returns the identifier of this provider.
+   */
+  public SkylarkProviderIdentifier id() {
+    return SkylarkProviderIdentifier.forKey(getKey());
+  }
 }