[Regression] ReferenceError: Can't find variable: Reflect

[haoqunjiang]

See vuejs/vue-cli#2733 for reproduction.
This regression is introduced in d570fa3#diff-5bb099174f678e4d728de8e607013b3aR5

The correct way to detect Reflect support should be:

return typeof Reflect !== 'undefined' && Reflect.defineMetadata

[ktsn]

Thanks! Fixed via v6.3.1 v6.3.2.