Unary minus and plus use wrong node Kind

FULL PRODUCT VERSION :
java version "9"
Java(TM) SE Runtime Environment (build 9+178)
Java HotSpot(TM) 64-Bit Server VM (build 9+178, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.1.7601]

A DESCRIPTION OF THE PROBLEM :
When a unary minus or plus is parsed, the Kind of the node is set to MINUS / PLUS (i.e. the BinaryTree constants) instead of UNARY_MINUS / UNARY_PLUS for UnaryTrees.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Inspect an expression like "-x" or "+1".

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
UNARY_MINUS

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
public static void main(String[] args) {
String script = "var a = -x;";
        DiagnosticListener listener = (Diagnostic diag) -> { System.err.println(diag.toString()); };

Parser parser = Parser.create("--language=es6", "--empty-statements");
Tree astRoot = parser.parse("unknown", script, listener);
astRoot.accept(new SimpleTreeVisitorES6<Boolean, Void>() {
@Override
public Boolean visitUnary(UnaryTree stmt, Void none) {
System.out.println(stmt.getKind());

return super.visitUnary(stmt, none);
}

}, null);
}

---------- END SOURCE ----------