Fix junit-platform-commons-java-9 subproject build

[nskvortsov]

Overview

Update Gradle script so that subproject junit-platform-commons-java-9 is really built with source, target and API of Java 9.

Before, two things were happening:

• --release compiler option was missing and all modules were compiler against JDK9 API
• javacRelease = 9 did not work for the subproject, because compiler settings were configured too early.

This was spotted while trying to import the project to IntelliJ IDEA https://youtrack.jetbrains.com/issue/IDEA-186344

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Definition of Done

• There are no TODOs left in the code
• N/A Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• N/A Change is covered by automated tests
• N/A Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes
• All continuous integration builds pass

[codecov]

Codecov Report

Merging #1288 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1288      +/-   ##
============================================
- Coverage     91.35%   91.31%   -0.04%     
+ Complexity     3123     3122       -1     
============================================
  Files           295      295              
  Lines          7646     7646              
  Branches        624      624              
============================================
- Hits           6985     6982       -3     
- Misses          484      485       +1     
- Partials        177      179       +2

Impacted Files	Coverage Δ	Complexity Δ
...jupiter/engine/execution/ExtensionValuesStore.java	87.14% <0%> (-4.29%)	21% <0%> (-1%)

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[sormuras]

Tried your changes locally and building it on console and importing the project into IDEA 2018.1 EAP works like a charm:

IntelliJ IDEA 2018.1 EAP (Community Edition)
Build #IC-181.3494.3, built on February 6, 2018
JRE: 1.8.0_152-release-1136-b5 amd64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
Windows 10 10.0

Does this fix also work with older versions of IDEA?

[nskvortsov]

Yes, it should be fixed for older versions.

[sormuras]

Thank you very much, @nskvortsov