added a comment - - edited
The release notes for v20131118 say "Move compiler to Java 7": https://groups.google.com/d/topic/closure-compiler-discuss/_T5Aai2sg68/discussion.
This release can still be built for Java 6 using ant jar -Dant.build.javac.source=1.6 -Dant.build.javac.target=1.6, but the current master makes use of Java 7 features in one class.
The Java 6/Java 7 support seems like it should be it's own issue. It's just how I discovered the problem, which is that running the bootstrap script doesn't give a consistent result. All of the other dependencies apart from closure-compiler have explicitly set versions, so I'm not sure I understand the reasoning. Shouldn't it download from central.maven.org with a version that matches project.clj?