Pass { childProcess, runLog } into post-startup callback function.

tools/isobuild/bundler.js

@@ -3166,12 +3166,17 @@ function bundle({
         // If we have a previous builder for this arch, and it's an arch
         // that we can safely build later (e.g. web.browser.legacy), then
         // schedule it to be built after the others.
-        postStartupCallbacks.push(childProcess => {
+        postStartupCallbacks.push(({
+          childProcess,
+          runLog,
+        }) => {
           const start = +new Date;
 
           // Build and write the target in one step.
           writeClientTarget(makeClientTarget(app, arch, { minifiers }));
 
+          // Return a Promise so that the caller has to wait for the
+          // childProcess.send to complete.
           return new Promise((resolve, reject) => {
             // Let the webapp package running in the child process know it
             // should regenerate the client program for this arch.
@@ -3183,13 +3188,15 @@ function bundle({
               if (error) {
                 reject(error);
               } else {
-                resolve(`Finished delayed build of ${arch} in ${
+                runLog.log(`Finished delayed build of ${arch} in ${
                   new Date - start
-                }ms`);
+                }ms`, { arrow: true });
+                resolve();
               }
             });
           });
         });
+
       } else {
         // Otherwise make the client target now, and write it below.
         targets[arch] = makeClientTarget(app, arch, {minifiers});

tools/runners/run-app.js

@@ -796,10 +796,11 @@ _.extend(AppRunner.prototype, {
         while (callbacks.length > 0) {
           const fn = callbacks.shift();
           try {
-            const message = Promise.await(fn(appProcess.proc));
-            if (message) {
-              runLog.log(message, { arrow: true });
-            }
+            Promise.await(fn({
+              // Miscellany that the callback might find useful.
+              childProcess: appProcess.proc,
+              runLog,
+            }));
           } catch (error) {
             buildmessage.error(error);
           }