Added fallback to drop builtin.

com.oracle.truffle.r.nodes.builtin/src/com/oracle/truffle/r/nodes/builtin/base/Drop.java

@@ -25,6 +25,7 @@ package com.oracle.truffle.r.nodes.builtin.base;
 import static com.oracle.truffle.r.runtime.builtins.RBehavior.PURE;
 import static com.oracle.truffle.r.runtime.builtins.RBuiltinKind.INTERNAL;
 
+import com.oracle.truffle.api.dsl.Fallback;
 import com.oracle.truffle.api.dsl.Specialization;
 import com.oracle.truffle.api.profiles.ConditionProfile;
 import com.oracle.truffle.r.nodes.builtin.RBuiltinNode;
@@ -132,4 +133,9 @@ public abstract class Drop extends RBuiltinNode {
             return RDataFactory.createStringVector(new String[]{(String) value}, true);
         }
     }
+
+    @Fallback
+    protected Object doDrop(Object x) {
+        return x;
+    }
 }