Fixes #1461. Existential assignment now correctly reports if it is a statement.

lib/nodes.js

@@ -1141,6 +1141,9 @@
       this.param = options && options.param;
     }
     Assign.prototype.children = ['variable', 'value'];
+    Assign.prototype.isStatement = function(o) {
+      return (o != null ? o.level : void 0) === LEVEL_TOP && __indexOf.call(this.context, "?") >= 0;
+    };
     Assign.prototype.assigns = function(name) {
       return this[this.context === 'object' ? 'value' : 'variable'].assigns(name);
     };

src/nodes.coffee

@@ -911,6 +911,9 @@ exports.Assign = class Assign extends Base
 
   children: ['variable', 'value']
 
+  isStatement: (o) ->
+    o?.level is LEVEL_TOP and "?" in @context
+
   assigns: (name) ->
     @[if @context is 'object' then 'value' else 'variable'].assigns name