Fix a runtime type error in the validator. (#1527)

I'm not sure why this wasn't causing us problems before.

lib/src/validator.dart

@@ -77,8 +77,9 @@ abstract class Validator {
 
     return Future.wait(validators.map((validator) => validator.validate()))
       .then((_) {
-      var errors = validators.expand((validator) => validator.errors);
-      var warnings = validators.expand((validator) => validator.warnings);
+      var errors = validators.expand((validator) => validator.errors).toList();
+      var warnings = validators.expand((validator) => validator.warnings)
+          .toList();
 
       if (!errors.isEmpty) {
         log.error("Missing requirements:");