fix(binder): adds more explicit error message thrown.

Author: rkirov

lib/core_dom/element_binder_builder.dart

@@ -93,7 +93,8 @@ class ElementBinderBuilder {
     if (annotation.map != null) {
       annotation.map.forEach((attrName, mapping) {
         if (mapping == null) {
-          throw "Null mapping value for '${attrName}' on annotation with selector '${annotation.selector}'.";
+          throw "Null mapping value for '${attrName}' on annotation with "
+              "selector '${annotation.selector}' with map '${annotation.map}'.";
         }
         Match match = _MAPPING.firstMatch(mapping);
         if (match == null) {