Update build plan to deal with required attributes

Only say() something, don't yet fail

src/Perl6/Metamodel/BUILDPLAN.nqp

@@ -18,6 +18,7 @@ role Perl6::Metamodel::BUILDPLAN {
     #   8 class attr_name code = call default value closure if needed, int attr
     #   9 class attr_name code = call default value closure if needed, num attr
     #   10 class attr_name code = call default value closure if needed, str attr
+    #   11 die if a required attribute is not present
     method create_BUILDPLAN($obj) {
         # First, we'll create the build plan for just this class.
         my @plan;
@@ -63,6 +64,13 @@ role Perl6::Metamodel::BUILDPLAN {
                 }
             }
         }
+
+        # Insure that any required attributes are set 
+        for @attrs {
+            if nqp::can($_, 'required') && $_.required {
+                @plan[+@plan] := [11, $obj, $_.name, 1];
+            }
+        } 
 
         # Install plan for this class.
         @!BUILDPLAN := @plan;

src/core/Mu.pm

@@ -179,6 +179,10 @@ my class Mu { # declared in BOOTSTRAP
                         nqp::atpos($task, 3)(self, $cur_value));
                 }
             }
+            elsif nqp::iseq_i($code, 11) {
+                # TODO throw a typed exception
+                nqp::say("eek, a required field!");
+            }
             else {
                 die "Invalid BUILDALLPLAN";
             }