diff --git a/packages/webapp/webapp_server.js b/packages/webapp/webapp_server.js
index f2373992d4..8e6265c524 100644
--- a/packages/webapp/webapp_server.js
+++ b/packages/webapp/webapp_server.js
@@ -245,6 +245,15 @@ var runWebAppServer = function () {
         };
         // Send the SourceMap header when the source file is served.
         staticFile.sourceMap = sourceMapRootUrl;
+
+        // Now register all the sources from this source map to be served, under
+        // the source map's URL.
+        _.each(item.sources, function (x, pathForSourceMap) {
+          staticFiles[sourceMapRootUrl + pathForSourceMap] = {
+            path: x.source,
+            cacheable: true
+          };
+        });
       }
       staticFiles[url.parse(item.url).pathname] = staticFile;
     }
@@ -293,7 +302,9 @@ var runWebAppServer = function () {
 
     // Tell the client where to find the source map for this file.
     if (info.sourceMap) {
-      res.setHeader('SourceMap', info.sourceMap);
+      // This should just be SourceMap, but slightly more browsers support the
+      // older X-SourceMap.
+      res.setHeader('X-SourceMap', info.sourceMap);
     }
 
     send(req, path.join(clientDir, info.path))
diff --git a/tools/bundler.js b/tools/bundler.js
index 10a2a7bbb2..51b4ecb350 100644
--- a/tools/bundler.js
+++ b/tools/bundler.js
@@ -902,7 +902,16 @@ _.extend(ClientTarget.prototype, {
             data: new Buffer(file.sourceMap.toString(), 'utf8')
           });
 
-        // XXX write sources too
+        manifestItem.sources = {};
+        _.each(file.sources, function (x, pathForSourceMap) {
+          manifestItem.sources[pathForSourceMap] = {
+            package: x.package,
+            sourcePath: x.sourcePath,
+            source: builder.writeToGeneratedFilename(
+              path.join('sources', pathForSourceMap),
+              { data: x.source })
+          };
+        });
       }
 
       manifest.push(manifestItem);