Fix sourcemapped column for a quoted key of object literal

lib/output.js

@@ -278,12 +278,29 @@ function OutputStream(options) {
 
     var add_mapping = options.source_map ? function(token, name) {
         try {
-            if (token) options.source_map.add(
+            if (token) {
+                var remainder = 0;
+                var offset = 0;
+                var tokenName;
+
+                tokenName = (!name && token.type == "name") ? token.value : name;
+
+                if(tokenName) {
+                  // Cast to a string, it might be a number
+                  tokenName = new String(tokenName);
+
+                  // Fixes offsets for quoted keys of object literals
+                  remainder = token.endpos - token.pos - tokenName.length;
+                  offset = remainder === 0 ? 0 : remainder/2;
+                }
+
+                options.source_map.add(
                   token.file || "?",
                   current_line, current_col,
-                token.line, token.col,
-                (!name && token.type == "name") ? token.value : name
+                  token.line, token.col + offset,
+                  tokenName
                 );
+            }
         } catch(ex) {
             AST_Node.warn("Couldn't figure out mapping for {file}:{line},{col} → {cline},{ccol} [{name}]", {
                 file: token.file,