refactor(probe): Remove use of NodeAttrs

Author: vicb

lib/mock/probe.dart

@@ -14,14 +14,15 @@ class Probe implements NgDetachAware {
   final Scope scope;
   final Injector injector;
   final Element element;
-  final NodeAttrs _attrs;
+  String _probeName;
 
-  Probe(this.scope, this.injector, this.element, this._attrs) {
-    scope.rootScope.context[_attrs['probe']] = this;
+  Probe(this.scope, this.injector, this.element) {
+    _probeName = element.attributes['probe'];
+    scope.rootScope.context[_probeName] = this;
   }
 
   void detach() {
-    scope.rootScope.context[_attrs['probe']] = null;
+    scope.rootScope.context[_probeName] = null;
   }
 
   /**