Unexpected syntax error with Flow

[rubennorte]

Bug Report

Current Behavior

Parsing this Flow code causes a syntax error in the latest release:

const foo = ([a: string, b: string]) => {}

Specifically the type casting of the destructured array elements.

But it's accepted in Flow: link.

Expected behavior/code

It shouldn't be a syntax error.

Environment

• Babel version(s): 7.1.6

Additional context/Screenshots
I think the issue was introduced in #8956.

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