The version number defaults in build.properties need to be bumped to reflect the new JDK 8 version. Specifically:

boot.javac.source = 7 (currently 6)
boot.javac.target = 7 (currently 6)
jdk.version = 1.8.0 (currently 1.7.0)
javac.source = 8 (currently 7)
javac.target = 8 (currently 7)

Currently, a development build of the compiler reports "1.7.0-internal" in response to the "-version" option.

It might be convenient to delay bumping the source/target tags until JDK 7 distributions become more widely available. Building a full JDK 8 already forces a 7 dependency, but for now, if development is limited to the langtools tree, it's still possible to build a bootstrap copy of the compiler with only JDK 6 on a system...