Add enforcement to android_binary to require *.pgcfg extension for ProGuard
configuration files, guarded by option "--enforce_proguard_file_extension".

RELNOTES: None
PiperOrigin-RevId: 248000326
diff --git a/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java b/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
index 97fffd3..b476a58 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
@@ -134,6 +134,7 @@
       Functions.<Set<String>>constant(ImmutableSet.<String>of());
 
   public static final PathFragment THIRD_PARTY_PREFIX = PathFragment.create("third_party");
+  public static final PathFragment EXPERIMENTAL_PREFIX = PathFragment.create("experimental");
   public static final String EXEC_COMPATIBLE_WITH_ATTR = "exec_compatible_with";
 
   /**