Google has finally done it! It has completely flipped the switch and has removed Dalvik from Android's master source code. This will pave the way for only one Android runtime, and that will be ART, which is expected to be pushed in the next major Android version, whenever that is.
This doesn't come as a shock since it was Google's explicit goal to have ART replace Dalvik ever since it debuted as a test option in Android 4.4. That said, so far ART and Dalvik have lived side by side each other and users have been able to switch between the two with the usual disclaimers of using a test product. Early this year, Google made ART the default runtime in one Android Open Source Project source code change, but Dalkvik remained as an alternative. Now, Google is completely yanking out the old runtime from the source code, leaving only ART behind. Of course, this is still in the master branch of the source code, which doesn't always match the next release, so when this change will actually descend on users is still anyone's guess.
The ART runtime is designed to bring many benefits to Android, for developers and users alike. It's most notable feature is the Ahead-of-Time (AOT) compilation, in contrast to Dalvik's Just-in-Time (JIT), which matches more closely with its Java roots. In layman's terms, compilation is the process of translating a program into executable code, a language that the machine, in this case the Android device, can understand and run. Dalvik performs this process when the program is run, hence the "just in time" moniker. ART, on the other hand, will compile programs when the app is installed or when the user switches from Dalvik to ART, saving up on startup time and resources, but at the expense of slightly slower installation times.
Theoretically, the transition from Dalvik to ART should be seamless and painless to most users. However, there might be some problems lurking beneath it that some, particular power users and tinkerers, will have to fight against. In particular, there is a possibility that rooting will become more difficult, if not totally impossible, with the switch to ART, as SuperSU developer Chainfire expressed last month. The good news is that the Android community is ever active in finding ways to give users their freedom and with the inescapable switch to ART, there will most likely be more interest and more hands working towards that end.