Fix compile_in_context with an immediate block

This situation could occur in the case of a begin-time EVAL that had a
code expression that contained an immediate QAST block. This led to the
failed $_ lookup reported in #2779, meaning that compilation of that
module no longer fails. Unfortunately, that isn't enough to make it
actually work, but it's a step closer.

src/Perl6/World.nqp

@@ -2783,16 +2783,22 @@ class Perl6::World is HLL::World {
         }
 
         # Compile it, set wrapper's static lexpad, then invoke the wrapper,
-        # which fixes up the lexicals.
+        # which fixes up the lexicals. For the duration of this compilation,
+        # we force the block type to declaration just to make sure we really
+        # only compile it, not run it (can happen if we're asking to compile
+        # an immediate block).
         my $compunit := QAST::CompUnit.new(
             :hll('perl6'),
             :sc(self.sc()),
             :compilation_mode(0),
             $wrapper
         );
         my $comp := nqp::getcomp('perl6');
+        my $orig_blocktype := $past.blocktype;
+        $past.blocktype('declaration');
         my $precomp := $comp.compile($compunit, :from<optimize>, :compunit_ok(1),
              :lineposcache($*LINEPOSCACHE));
+        $past.blocktype($orig_blocktype);
         my $mainline := $comp.backend.compunit_mainline($precomp);
         $mainline();