Description:
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When no parameters are supplied to MessageFormatter::formatMessage or $messageFormatter->format all placeholders are replaced with {0}. If at least one argument is supplied it works as expected.

Test script:
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$pattern = 'Let us do {test} now!';
echo MessageFormatter::formatMessage('en_US', $pattern, array());

$formatter = new MessageFormatter('en_US', $pattern);
echo $formatter->format(array());

Expected result:
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Let us do {test} now!

Actual result:
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Let us do {0} now!