Incremental compiler doesn't detect JDK change

[eed3si9n]

1. compileOrder := CompileOrder.JavaThenScala
2. I'm using all Java project
3. Compile using Java 7
4. Exit sbt, and change to Java 6
5. Compile again

It looked like the compiler didn't noticed that the underlying JDK was changed.
I kept getting "Unsupported major.minor version 51.0" on the consuming side.

[retronym]

Related: sbt/zinc#825

[retronym]

I suggest that previous products (ie generated class files) should be considered invalid after a JDK change.

[eed3si9n]

sbt does initialize extraIncOptions as follows:

sbt/main/src/main/scala/sbt/Defaults.scala

Line 190 in 9e98b6d

extraIncOptions :== Seq("JAVA_CLASS_VERSION" -> sys.props("java.class.version")),