An implementation of JavacTask that implements only the frontend portions of the task,
using only the parse phase of the underlying compiler and without requiring a complete classpath.
This effectively does the same thing as the Enter phase of the compiler (see
http://openjdk.java.net/groups/compiler/doc/compilation-overview/index.html), but applying
heuristics when dependencies are missing. This necessarily requires some assumptions to be made
about references to symbols defined in those dependencies. See the documentation of com.facebook.buck.jvm.java.abi.source for details.