Fix LTA error on List.push/pop/unshift/shift

pippo++ for pointing this out

src/core/List.pm

@@ -3,6 +3,7 @@ my class X::TypeCheck { ... }
 my class X::TypeCheck::Splice { ... }
 my class X::Cannot::Lazy { ... }
 my class X::Cannot::Empty { ... }
+my class X::Immutable { ... }
 my role Supply { ... }
 
 my sub combinations($n, $k) {
@@ -836,6 +837,19 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
         nqp::push_s($rsa, '...') if $infinite;
         nqp::p6box_s(nqp::join(nqp::unbox_s($separator.Str), $rsa))
     }
+
+    method push(|) {
+        X::Immutable.new( method => 'push',    typename => 'List' ).throw;
+    }
+    method pop(|) {
+        X::Immutable.new( method => 'pop',     typename => 'List' ).throw;
+    }
+    method shift(|) {
+        X::Immutable.new( method => 'shift',   typename => 'List' ).throw;
+    }
+    method unshift(|) {
+        X::Immutable.new( method => 'unshift', typename => 'List' ).throw;
+    }
 }
 
 # The , operator produces a List.