refactor(dart/transform): Add warning about issue angular#1747

Add a more descriptive warning about the lack of support for `Directive`
dependency aliases and a pointer to more information.

Author: Tim Blasi

modules/angular2/src/transform/template_compiler/view_definition_creator.dart

@@ -254,7 +254,11 @@ class _TemplateExtractVisitor extends Object with RecursiveAstVisitor<Object> {
       if (_metadataMap.containsKey(name)) {
         viewDef.directives.add(_metadataMap[name]);
       } else {
-        logger.warning('Could not find Directive entry for $name');
+        logger.warning('Could not find Directive entry for $name. '
+            'Please be aware that reusable, pre-defined lists of Directives '
+            '(aka "directive aliases") are not yet supported and will cause '
+            'your application to misbehave. '
+            'See https://github.com/angular/angular/issues/1747 for details.');
       }
     }
   }