Broken compiled code for GetItemExpression on PHP 5.3

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Assuming the foo variable is a container parameter, the expression foo['bar'] will be compiled (once #110 allows to compile it) to $context['container']->getParameter('foo_param')['bar']. This is broken on PHP 5.3 which does not support array dereferencing.

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The solution used in the Symfony 2.4+ ExpressionLanguage component is to throw an exception at compile-time saying it cannot be compiled on 5.3