better support for multiple input files:

- use a single AST_Toplevel node for all files
- keep original source filename in the tokens

lib/output.js

@@ -54,7 +54,8 @@ function OutputStream(options) {
         max_line_len  : 32000,
         ie_proof      : true,
         beautify      : true,
-        source_map    : null
+        source_map    : null,
+        in_source_map : null
     });
 
     var indentation = 0;
@@ -245,6 +246,7 @@ 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