Add support for Java 11 and 12 class files with "preview features"

[Godin]

See JEP 12 (Preview Language and VM Features). I'm not aware of any preview features for JDK 11, however --enable-preview flag is available in JDK 11 EA. And as of today there is at least one preview feature scheduled for JDK 12 - JEP 325 (Switch Expressions).

---

Before this change:

$ cat <<END > Example.java
class Example {
  public static void main(String[] args) {
  }
}
END

$ javac --enable-preview --release 12 Example.java

$ java --enable-preview -javaagent:before/lib/jacocoagent.jar Example

$ java -jar before/lib/jacococli.jar execinfo jacoco.exec            
[INFO] Loading exec file jacoco.exec.
CLASS ID         HITS/PROBES   CLASS NAME
Session "godin-laptop2-5141427c": Sat Aug 18 23:39:49 CEST 2018 - Sat Aug 18 23:39:49 CEST 2018
4091363f4c5d7293    1 of   2   Example

$ java -jar before/lib/jacococli.jar report jacoco.exec --classfiles Example.class
[INFO] Loading execution data file /tmp/jacoco/jacoco.exec.
[INFO] Analyzing 0 classes.

After this change:

$ java -jar after/lib/jacococli.jar report jacoco.exec --classfiles Example.class
[INFO] Loading execution data file /tmp/jacoco/jacoco.exec.
[INFO] Analyzing 1 classes.

[marchof]

Add "... including JEP 12 preview versions"?

[Godin]

@marchof added

, including JEP 12 "preview features"