Connect phone.
adb pull /data/property/persist.sys.dalvik.vm.lib
Open the file in notepad or whatever.
Replace the text libart.so with libdvm.so and you've got Dalvik.
Replace the libdvm.so ... yeah, you get it, vice versa.
adb push persist.sys.dalvik.vm.lib /data/property/
Boom! ... well, hopefully not. ;)
Source