Linked message does not resolve if one of the key in path contains hyphen (-)

[kashiif]

vue & vue-i18n version

2.3.3, 7.0.0

Reproduction Link

https://github.com/kashiif/vue-intl-test

Steps to reproduce

1. Run the app in above repo
2. See heading Linked Message (Item 2 fails)

What is Expected?

The second item under heading Linked Message (Item 2 fails) should have displayed:

**topLevel.linked2:** linked2: error-message (Hyphen)

What is actually happening?

If a linked message is defined as

{
topLevel: {
  'error-message': '...',
},
linked2: 'linked2: @:topLevel.error-message',
}

with a hyphen in the path, it does not resolve to correct translation. Instead, it shows:

linked2: null-message