Avoid breaking backwards compatibility for asset URLs.

packages/http/httpcall_tests.js

@@ -487,7 +487,7 @@ if (Meteor.isServer) {
 
       function do_test(path, code, match) {
         const prefix = Meteor.isModern
-          ? "/__browser"
+          ? "" // No prefix for web.browser (modern).
           : "/__browser.legacy";
 
         HTTP.get(url_base() + prefix + path, {

packages/modern-browsers/TODO.md

@@ -12,6 +12,9 @@
 - [x] Really vet the set of imported `core-js` polyfills based on known
       minimum versions imposed by `setMinimumBrowserVersions`.
 
+- [x] Make sure the new url prefixes aren't too disruptive for public
+      assets like images.
+
 - [ ] Create an `isobuild:web-browser-legacy` pseudopackage.
 
 - [ ] Add tests to the `modules` test app.

tools/isobuild/bundler.js

@@ -576,7 +576,20 @@ class File {
     this.url = null;
 
     // A prefix that will be prepended to this.url.
-    if (options.arch.startsWith("web.")) {
+    if (options.arch.startsWith("web.") &&
+        // Use /__browser.legacy/... as a prefix for web.browser.legacy
+        // URLs, but avoid adding a special prefix to resource URLs for
+        // modern browsers. Though boilerplate-generator will happily use
+        // whatever URLs we invent here, it's important that assets like
+        // images are available from predictable URLs (without any
+        // arch-specific prefixes), since humans might use those URLs in
+        // hand-written code. Moreover, non-JS assets are typically the
+        // same for both modern and legacy browsers, so the URL prefix
+        // doesn't actually make a difference. In the unlikely event that
+        // someone adds different assets with the same path to web.browser
+        // and web.browser.legacy, the legacy version can always be
+        // fetched from the /__browser.legacy/... URL.
+        options.arch !== "web.browser") {
       this.urlPrefix = "/__" +
         options.arch.split(".").slice(1).join(".");
     } else {

tools/tests/compiler-plugins.js

@@ -393,7 +393,10 @@ selftest.define("compiler plugins - compiler throws", () => {
 });
 
 function checkModernAndLegacyUrls(path, test) {
-  test(getUrl("http://localhost:3000/__browser" + path));
+  if (! path.startsWith("/")) {
+    path = "/" + path;
+  }
+  test(getUrl("http://localhost:3000" + path));
   test(getUrl("http://localhost:3000/__browser.legacy" + path));
 }
 

tools/tests/modules.js

@@ -11,6 +11,7 @@ function startRun(sandbox) {
   run.match("myapp");
   run.match("proxy");
   run.tellMongo(MONGO_LISTENING);
+  run.waitSecs(20);
   run.match("MongoDB");
   return run;
 };
@@ -94,6 +95,9 @@ selftest.define("modules - import chain for packages", () => {
 });
 
 function checkModernAndLegacyUrls(path, test) {
-  test(getUrl("http://localhost:3000/__browser" + path));
+  if (! path.startsWith("/")) {
+    path = "/" + path;
+  }
+  test(getUrl("http://localhost:3000" + path));
   test(getUrl("http://localhost:3000/__browser.legacy" + path));
 }