diff --git a/tools/build-plugin.js b/tools/build-plugin.js
index f8ffb4bc8e..2591177089 100644
--- a/tools/build-plugin.js
+++ b/tools/build-plugin.js
@@ -60,8 +60,11 @@ _.extend(exports.SourceProcessor.prototype, {
 // Represents a set of SourceProcessors available in a given package. They may
 // not have conflicting extensions or filenames.
 export class SourceProcessorSet {
-  constructor(myPackageDisplayName,
-              { hardcodeJs, singlePackage, allowConflicts }) {
+  constructor(myPackageDisplayName, {
+    hardcodeJs,
+    singlePackage,
+    allowConflicts,
+  } = {}) {
     // For error messages only.
     this._myPackageDisplayName = myPackageDisplayName;
     // If this represents the SourceProcessors *registered* by a single package
@@ -206,13 +209,6 @@ export class SourceProcessorSet {
     // don't use iteration functions, so we can return (and start at #1)
     for (let i = 1; i < parts.length; i++) {
       const extension = parts.slice(i).join('.');
-      // We specially handle 'js' in the build tool, because you can't provide a
-      // plugin to handle 'js' files, because the plugin would need to be built
-      // with JavaScript itself!  Places that hardcode JS are tagged with
-      // #HardcodeJs.
-      if (this._hardcodeJs && extension === 'js') {
-        return new SourceClassification('extension', {extension});
-      }
 
       if (this._byExtension.hasOwnProperty(extension)) {
         return new SourceClassification('extension', {
@@ -222,6 +218,13 @@ export class SourceProcessorSet {
         });
       }
 
+      if (this._hardcodeJs && extension === 'js') {
+        // If there is no special sourceProcessor for handling a .js file,
+        // we can still classify it as extension/js, only without any
+        // source processors. #HardcodeJs
+        return new SourceClassification('extension', {extension});
+      }
+
       if (this._legacyHandlers.hasOwnProperty(extension)) {
         const legacy = this._legacyHandlers[extension];
         if (legacy.archMatching &&
@@ -251,17 +254,28 @@ export class SourceProcessorSet {
   appReadDirectoryOptions(arch) {
     const include = [];
     const names = [];
+    let addedJs = false;
 
     function addExtension(ext) {
       include.push(new RegExp('\\.' + utils.quotemeta(ext) + '$'));
+      if (ext === 'js') {
+        addedJs = true;
+      }
     }
+
     _.each(this._byExtension, (sourceProcessors, ext) => {
       if (sourceProcessors.some(sp => sp.relevantForArch(arch))) {
         addExtension(ext);
       }
     });
     Object.keys(this._legacyHandlers).forEach(addExtension);
-    this._hardcodeJs && addExtension('js');
+
+    if (this._hardcodeJs && ! addedJs) {
+      // If there is no sourceProcessor for handling .js files, we still
+      // want to make sure they get picked up when we're reading the
+      // contents of app directories. #HardcodeJs
+      addExtension('js');
+    }
 
     _.each(this._byFilename, (sourceProcessors, filename) => {
       if (sourceProcessors.some(sp => sp.relevantForArch(arch))) {
diff --git a/tools/compiler-plugin.js b/tools/compiler-plugin.js
index f0f16ff7fd..dae30b97c6 100644
--- a/tools/compiler-plugin.js
+++ b/tools/compiler-plugin.js
@@ -264,29 +264,25 @@ var ResourceSlot = function (unibuildResourceInfo,
   self.sourceProcessor = sourceProcessor;
   self.packageSourceBatch = packageSourceBatch;
 
-  if (self.inputResource.type === "source" &&
-      self.inputResource.extension === "js") {
-    // #HardcodeJs
+  if (self.inputResource.type === "source") {
     if (sourceProcessor) {
-      throw Error("sourceProcessor found for js source? " +
-                  JSON.stringify(unibuildResourceInfo));
-    }
-    self.addJavaScript({
-      // XXX it's a shame to keep converting between Buffer and string, but
-      // files.convertToStandardLineEndings only works on strings for now
-      data: self.inputResource.data.toString('utf8'),
-      path: self.inputResource.path,
-      hash: self.inputResource.hash,
-      bare: self.inputResource.fileOptions &&
-        (self.inputResource.fileOptions.bare ||
-         // XXX eventually get rid of backward-compatibility "raw" name
-         // XXX COMPAT WITH 0.6.4
-         self.inputResource.fileOptions.raw)
-    });
-  } else if (self.inputResource.type === "source") {
-    if (! sourceProcessor) {
-      throw Error("no sourceProcessor for source? " +
-                  JSON.stringify(unibuildResourceInfo));
+      // If we have a sourceProcessor, it will handle the adding of the
+      // final processed JavaScript.
+    } else if (self.inputResource.extension === "js") {
+      // If there is no sourceProcessor for a .js file, add the source
+      // directly to the output. #HardcodeJs
+      self.addJavaScript({
+        // XXX it's a shame to keep converting between Buffer and string, but
+        // files.convertToStandardLineEndings only works on strings for now
+        data: self.inputResource.data.toString('utf8'),
+        path: self.inputResource.path,
+        hash: self.inputResource.hash,
+        bare: self.inputResource.fileOptions &&
+          (self.inputResource.fileOptions.bare ||
+           // XXX eventually get rid of backward-compatibility "raw" name
+           // XXX COMPAT WITH 0.6.4
+           self.inputResource.fileOptions.raw)
+      });
     }
   } else {
     if (sourceProcessor) {
@@ -395,10 +391,7 @@ var PackageSourceBatch = function (unibuild, processor) {
     let sourceProcessor = null;
     if (resource.type === "source") {
       var extension = resource.extension;
-      if (extension === 'js') {
-        // #HardcodeJs In this case, we just leave buildPlugin null; it is
-        // specially handled by ResourceSlot too.
-      } else if (extension === null) {
+      if (extension === null) {
         const filename = files.pathBasename(resource.path);
         sourceProcessor = sourceProcessorSet.getByFilename(filename);
         if (! sourceProcessor) {
@@ -411,7 +404,9 @@ var PackageSourceBatch = function (unibuild, processor) {
         }
       } else {
         sourceProcessor = sourceProcessorSet.getByExtension(extension);
-        if (! sourceProcessor) {
+        // If resource.extension === 'js', it's ok for there to be no
+        // sourceProcessor, since we #HardcodeJs in ResourceSlot.
+        if (! sourceProcessor && extension !== 'js') {
           buildmessage.error(
             `no plugin found for ${ resource.path } in ` +
             `${ unibuild.pkg.displayName() }; a plugin for *.${ extension } ` +
diff --git a/tools/isopack.js b/tools/isopack.js
index f19a08d786..ad3f280a6b 100644
--- a/tools/isopack.js
+++ b/tools/isopack.js
@@ -1149,7 +1149,8 @@ _.extend(Isopack.prototype, {
           // If we're going to write a legacy prelink file later, track the
           // original form of the resource object (with the source in a Buffer,
           // etc) instead of the later version.  #HardcodeJs
-          if (writeLegacyBuilds && resource.type === "source" &&
+          if (writeLegacyBuilds &&
+              resource.type === "source" &&
               resource.extension == "js") {
             jsResourcesForLegacyPrelink.push({
               data: resource.data,