RFR: Initial commit of data plugin

[Leonid Kuskov]

On Wed, 11 Aug 2021 02:55:19 GMT, Alexandre Iline <shurailine at openjdk.org> wrote:

> Initial commit of data coverage JCov plugin. See the readme and tests.

Marked as reviewed by lkuskov (Committer).

LGFM

plugins/data_coverage/src/openjdk/jcov/data/Env.java line 35:

> 33: import static java.util.stream.Collectors.toMap;
> 34: 
> 35: public abstract class Env<T> {

Please comment classes to clarify their purposes. 
Why this class is needed, System.Properties doesn't work?

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PR: https://git.openjdk.java.net/jcov/pull/16