for certain nodes that we invent we might not have a original source file to

map from, so just use "?".  and in any case, don't fail hard when addMapping throws.

lib/output.js

@@ -248,12 +248,23 @@ function OutputStream(options) {
     };
 
     var add_mapping = options.source_map ? function(token, name) {
-        options.source_map.add(
-            token.file,
-            current_line, current_col,
-            token.line, token.col,
-            (!name && token.type == "name") ? token.value : name
-        );
+        try {
+            if (token) options.source_map.add(
+                token.file || "?",
+                current_line, current_col,
+                token.line, token.col,
+                (!name && token.type == "name") ? token.value : name
+            );
+        } catch(ex) {
+            AST_Node.warn("Couldn't figure out mapping for {file}:{line},{col} → {cline},{ccol} [{name}]", {
+                file: token.file,
+                line: token.line,
+                col: token.col,
+                cline: current_line,
+                ccol: current_col,
+                name: name || ""
+            })
+        }
     } : noop;
 
     function get() {