Instance type of VueProxy allows any field access when props is not specified

[ktsn]

version: 0.3.2

Reproduction

https://codesandbox.io/s/vuecomposition-api-type-issue-dc492

Expected Behavior

comp.foo should be error as there are no field type foo in the component.

Actual Behavior

No error is provided.

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This behavior would be a problem if the users enable template type checking which Vetur offers.
As Vetur uses component instance type to check expression, it cannot provide diagnostic message even if an accessed field is defined on the component.