Fix copy'n'paste error: it is not polymorphic at that point

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Add source coordinates to missing nodes

- implicit self return node was missing source info
- variable reads were missing source info

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Add source info to Method node

- and added toString for better debugging of Graal/Truffle issues

Signed-off-by: Stefan Marr <git@stefan-marr.de>