fix for class names starting with U (will be \U and will be parsed as unicode and will fail in Rascal on readTextValueString

Example of failure:

rascal>readTextValueString(#str, "\"Mockery\\Undefined\"");
|rascal://ValueIO|(1462,241,<48,0>,<51,63>): Java("NumberFormatException","For input string: \"ndefin\"")
	at java.lang.NumberFormatException.forInputString(|file:///NumberFormatException.java|(0,0,<65,0>,<65,0>))
	at java.lang.Integer.parseInt(|file:///Integer.java|(0,0,<492,0>,<492,0>))
	at org.eclipse.imp.pdb.facts.io.StandardTextReader.readString(|file:///StandardTextReader.java|(0,0,<676,0>,<676,0>))
	at org.eclipse.imp.pdb.facts.io.StandardTextReader.readValue(|file:///StandardTextReader.java|(0,0,<135,0>,<135,0>))
	at org.eclipse.imp.pdb.facts.io.StandardTextReader.read(|file:///StandardTextReader.java|(0,0,<98,0>,<98,0>))
	at org.rascalmpl.library.Prelude.readTextValueString(|file:///Prelude.java|(0,0,<3342,0>,<3342,0>))
	at readTextValueString(|stdin:///|(26,24,<1,26>,<1,50>))