Do not attempt evaluating class expressions

Broadly the same as function expressions - these actually are statements but
we're limited by the inheritance tree.

Fixes #1044

lib/compress.js

@@ -977,6 +977,9 @@ merge(Compressor.prototype, {
         def(AST_Arrow, function() {
             throw def;
         });
+        def(AST_Class, function() {
+            throw def;
+        });
         function ev(node, compressor) {
             if (!compressor) throw new Error("Compressor must be passed");