I thought Flex 4.5 is just bug fixing release for Flex 4. Any new API or new functionality?
I think mostly the update was related to tooling (Flash Builder 4.5 vs 4.0), but here is the list of updates made to the SDK:
Sorry, I ment to post this link: http://www.adobe.com/devnet/flex/articles/introducing-flex45sdk.html
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