XML parser reverses attribute order

[biswanaths]

When parsing XML and then writing it out again, the order of attributes is reversed.

This prevents certain use cases where one wants to modify an XML file and have a minimal amount of changes. E.g. adding a new element to a file causes all existing attributes to get reversed.

The cause is here:

https://github.com/scala/scala/blob/81e3121eb09010375783d13e8c4c853686a34ffd/src/library/scala/xml/parsing/MarkupParser.scala#L302

aMap: MetaData is built by prepending new attributes to the linked list, causing the reversal.

The attached patch adds a failing test case. (It failed when I wrote it originally, now I can't test it because the build fails).

Imported from : SI-6341