throw a syntax error if you try to use the fat arrow to define a class constructor.

lib/nodes.js

@@ -739,6 +739,9 @@
         pvar = _e[0];
         func = _e[1];
         if (pvar && pvar.base.value === 'constructor' && func instanceof CodeNode) {
+          if (func.bound) {
+            throw new Error("cannot define a constructor as a bound function.");
+          }
           func.name = className;
           func.body.push(new ReturnNode(literal('this')));
           this.variable = new ValueNode(this.variable);