Fix bug in loops.

It was still popping the stack for the body even though loop
bodies are statements now.

src/wren_core.c

@@ -212,6 +212,12 @@ DEF_NATIVE(list_subscript)
   return list->elements[index];
 }
 
+DEF_NATIVE(null_toString)
+{
+  // TODO(bob): Intern this string or something.
+  return wrenNewString(vm, "null", 4);
+}
+
 DEF_NATIVE(num_abs)
 {
   return NUM_VAL(fabs(AS_NUM(args[0])));
@@ -471,6 +477,7 @@ void wrenInitializeCore(WrenVM* vm)
   NATIVE(vm->listClass, "[ ]", list_subscript);
 
   vm->nullClass = defineClass(vm, "Null", vm->objectClass);
+  NATIVE(vm->nullClass, "toString", null_toString);
 
   vm->numClass = defineClass(vm, "Num", vm->objectClass);
   NATIVE(vm->numClass, "abs", num_abs);

src/wren_vm.c

@@ -5,8 +5,9 @@
 #include "wren.h"
 #include "wren_common.h"
 #include "wren_compiler.h"
-#include "wren_vm.h"
 #include "wren_core.h"
+#include "wren_vm.h"
+#include "wren_debug.h"
 
 WrenVM* wrenNewVM(WrenReallocateFn reallocateFn)
 {
@@ -939,10 +940,6 @@ Value interpret(WrenVM* vm, Value function)
 
     CASE_CODE(LOOP):
     {
-      // The loop body's result is on the top of the stack. Since we are
-      // looping and running the body again, discard it.
-      POP();
-
       // Jump back to the top of the loop.
       int offset = READ_ARG();
       ip -= offset;

src/wren_vm.h

@@ -186,6 +186,7 @@ struct WrenVM
   Value unsupported;
 
   SymbolTable globalSymbols;
+
   // TODO(bob): Using a fixed array is gross here.
   Value globals[MAX_SYMBOLS];