Don't pretend to evaluate lambdas

Fix #255

lib/compress.js

@@ -611,7 +611,7 @@ merge(Compressor.prototype, {
             // inherits from AST_Statement; however, an AST_Function
             // isn't really a statement.  This could byte in other
             // places too. :-( Wish JS had multiple inheritance.
-            return [ this ];
+            throw def;
         });
         function ev(node) {
             return node._eval();