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Add async xmlwriter methods #636

Job	Run time
Build and Test (debug, net35, windows-latest)	8m 56s
Event File	2s
Build and Test (debug, net40, windows-latest)	7m 55s
Validate generated files	3m 56s
Build and Test (debug, All, windows-latest)	13m 34s
Build and Test (debug, All, ubuntu-latest)	5m 33s
Build and Test (debug, All, macos-latest)	4m 48s
Build and Test (release, net35, windows-latest)	8m 28s
Build and Test (release, net40, windows-latest)	7m 18s
Build and Test (release, All, windows-latest)	7m 58s
Build and Test (release, All, ubuntu-latest)	3m 4s
Build and Test (release, All, macos-latest)	3m 27s
	1h 14m 59s

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