Use 'index' to access the enum value index. (#2865)

Co-authored-by: Sam Rawlins <[email protected]>

Author: scheglov

lib/src/model/model_element.dart

@@ -179,7 +179,7 @@ abstract class ModelElement extends Canonicalization
     if (e is FieldElement) {
       if (enclosingContainer == null) {
         if (e.isEnumConstant) {
-          var index = e.computeConstantValue().getField(e.name).toIntValue();
+          var index = e.computeConstantValue().getField('index').toIntValue();
           newModelElement =
               EnumField.forConstant(index, e, library, packageGraph, getter);
         } else if (e.enclosingElement is ExtensionElement) {