assertDictionaryItem()function

assertDictionaryItem(dict: ImmutableDictionary<T>, key: string, caller: AnyCaller = assertDictionaryItem): asserts key is string

Param	Type
`dict`	`ImmutableDictionary<T>`	The dictionary to assert against. required
`key`	`string`	The key to assert is an own prop. required
`caller`	`AnyCaller`	Function to attribute a thrown error to (defaults to `assertDictionaryItem` itself). Defaults to `assertDictionaryItem`

Return
`asserts key is string`

Throws
`RequiredError`	If `key` is not an own prop of `dict`.

Assert that an unknown value is the key for an own prop of a dictionary.