Throwing proper error when "constructor" is not a function within a class body.

lib/nodes.js

@@ -821,6 +821,9 @@
         pvar = prop.variable;
         if (pvar && pvar.base.value === 'constructor') {
           func = prop.value;
+          if (!(func instanceof CodeNode)) {
+            throw new Error("'" + (pvar.base.value) + "' must be a function in 'class " + (this.variable.base.value) + "'.");
+          }
           func.body.push(new ReturnNode(literal('this')));
           constructor = new AssignNode(this.variable, func);
         } else {