Added @ on Numbers; fixed js import error message.

Made the js code generator generate code to access StandardPrelude.
Added Point2D to StandardPrelude.   Put code in both c and js
libraries to implement Points.  This needs to be extended to provide
more methods.
Corrected golden output for t150.

Corrected file name, since this is not a failing test.
Deleted test 150.

This test gives a run-time error in JavaScript and a compile-time
error in C.  Is it worth trying to mnake it pass in both?