If you are a bit of geek and use an Android kitkat device, you will be aware that with the release of Android 4.4 kitkat, Google introduced ART compiler as an experimental feature along with the existing Dalvik. Dalvik is still the default runtime compiler and ART can be enabled by changing a small setting in the Developer options.
It now appears that the next major version of Android will come up with default ART runtimetime compiler and Dalvik will be removed. This information is based on two commits made recently (ommits 98553 and 98618) to the AOSP master branch which show Dalvik getting the axe and ART being set as the default.
At the moment, shifting to ART from Dalvik often poses problems for users as the feature is still experimental and hopefully when it is set as default with the next major release of Android, it will be stable and smooth.
Source : XDA