better support for multiple input files:

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

lib/sourcemap.js

@@ -51,27 +51,29 @@ function SourceMap(options) {
         file       : options.file,
         sourceRoot : options.root
     });
-    var current_source = null;
-    function add(gen_line, gen_col, orig_line, orig_col, name) {
-        // AST_Node.warn("Mapping in {file}: {orig_line},{orig_col} → {gen_line},{gen_col} ({name})", {
-        //     orig_line : orig_line,
-        //     orig_col  : orig_col,
-        //     gen_line  : gen_line,
-        //     gen_col   : gen_col,
-        //     file      : current_source,
-        //     name      : name
-        // });
+    function add(source, gen_line, gen_col, orig_line, orig_col, name) {
         generator.addMapping({
             generated : { line: gen_line + 1, column: gen_col },
             original  : { line: orig_line + 1, column: orig_col },
-            source    : current_source,
+            source    : source,
             name      : name
         });
     };
     return {
         add        : add,
-        set_source : function(filename) { current_source = filename },
         get        : function() { return generator },
         toString   : function() { return generator.toString() }
-    }
+    };
+};
+
+function SourceMapInput(map) {
+    map = new MOZ_SourceMap.SourceMapConsumer(map);
+    return {
+        info: function(line, col) {
+            return map.originalPositionFor({
+                line: line,
+                column: col
+            });
+        }
+    };
 };