minor improvement to use stripSuffix in DotGraph
short-circuit the macro check in the incremental compiler if the source isn't a Scala source file
test for #678
Include jars from java.ext.dirs in incremental classpath.  Fixes #678.

Ideally, we wouldn't need to construct the classpath ourselves and instead
reuse the classpath construction code from a compiler (scalac or javac).
However, we need to ensure that we only call the compiler when needed.
Because we need to construct the classpath even when compilation might
not happen, we have to duplicate classpath construction.