lib/jrt-fs.jar is incompatible with Java 8

A program running on Java 8 should be able to use the library to peek inside Java 9 (JEP 220 "Modular Run-Time Images" [1], section "New URI scheme ...").

Steps to reproduce:
1. Download an attached program
2. `apt install openjdk-8-jdk openjdk-9-jdk`
3. `/usr/lib/jvm/java-8-openjdk-amd64/bin/javac ReadJrt.java`
4. `/usr/lib/jvm/java-8-openjdk-amd64/bin/java ReadJrt /usr/lib/jvm/java-9-openjdk-amd64`

Expected result:
The program lists root directories of a runtime image (/packages, /modules) and finishes normally.

Actual result:
The program fails with the following exception:
Exception in thread "main" java.lang.NoSuchMethodError: java.nio.ByteBuffer.limit(I)Ljava/nio/ByteBuffer;
 at jdk.internal.jimage.BasicImageReader.slice(BasicImageReader.java:211)
 at jdk.internal.jimage.BasicImageReader.intBuffer(BasicImageReader.java:218)
 at jdk.internal.jimage.BasicImageReader.<init>(BasicImageReader.java:152)
 at jdk.internal.jimage.ImageReader$SharedImageReader.<init>(ImageReader.java:224)
 at jdk.internal.jimage.ImageReader$SharedImageReader.open(ImageReader.java:238)
 at jdk.internal.jimage.ImageReader.open(ImageReader.java:67)
 at jdk.internal.jimage.ImageReader.open(ImageReader.java:71)
 at jdk.internal.jrtfs.SystemImage.open(SystemImage.java:59)
 at jdk.internal.jrtfs.JrtFileSystem.<init>(JrtFileSystem.java:90)
 at jdk.internal.jrtfs.JrtFileSystemProvider.newFileSystem(JrtFileSystemProvider.java:108)
 at java.nio.file.FileSystems.newFileSystem(FileSystems.java:336)
 at ReadJrt.main(ReadJrt.java:33)

Probable reason:
Classes in lib/jrt-fs.jar were compiled by Java 9 with options "-source 8"/"target 8". They should be compiled with "--release 8" option instead (or by Java 8).

[1] http://openjdk.java.net/jeps/220

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: openjdk-9-jre-headless 9~b181-4
ProcVersionSignature: Ubuntu 4.13.0-16.19-generic 4.13.4
Uname: Linux 4.13.0-16-generic x86_64
ApportVersion: 2.20.7-0ubuntu3
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Tue Oct 24 20:06:01 2017
InstallationDate: Installed on 2017-09-29 (25 days ago)
InstallationMedia: Ubuntu 17.10 "Artful Aardvark" - Alpha amd64 (20170926)
SourcePackage: openjdk-9
UpgradeStatus: No upgrade log present (probably fresh install)