handle [null] in template-repeat

[sigmundch]

This seems to be allowed in JS:

<template repeat="{{[null]}}">
{{id}}
</template>

The repeat will not render the null entry, but in Dart we seem to do so.

[jmesserly]

this should go away when we fix https://code.google.com/p/dart/issues/detail?id=18893

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Added Duplicate label.
Marked as being merged into #18893.