repeat.for should check if it gets an iterable collection

[zakjan]

class TestViewModel {
    value: number;
    nullableCollection: string[] | undefined;
}

<template>
    <template repeat.for="item of value">${item}</template> <!-- should warn -->
    <template repeat.for="item of nullableCollection">${item}</template> <!-- should warn -->
    <template if.bind="nullableCollection">
        <template repeat.for="item of nullableCollection">${item}</template> <!-- ok -->
    </template>
</template>