ExtentedStringLiteral can be supported apparently

This statement throwing an exception does not seem to be needed since parseLiteralExpressionsAsConstants is forced to false is the compiler options of JDTBatchCompiler.
And by the way, removing this exception throwing does not break any test.
getDefaultExpression() returns the expression that initializes the variable. Since the expression is in this case a String, you get the content of the string enclosed in the double quotes.