Summary
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Add `-source` and `-target` options to `javac` corresponding to JDK 12.

Problem
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Part of starting development on a new JDK is adding .new  `-source` and `-target` options to `javac`. These options are used in the compilation of the JDK sources.


Solution
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Add support for  `javac -source 12 ...` and `javac -target 12 ...`.

The new value of -source/-target is also made the default.

Note: following the precedent set in JDK 11, "1.12" is *not* treated as an alias for "12".

Specification
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No update is needed in `javac --help`; the conceptual man page should reflect the increased range of `-source`/`-target` values.