diff --git a/tools/packages.js b/tools/packages.js
index 711f54c575..904bb74b6a 100644
--- a/tools/packages.js
+++ b/tools/packages.js
@@ -20,7 +20,7 @@ var sourcemap = require('source-map');
 // unipackage/slice changes, but this version (which is build-tool-specific) can
 // change when the the contents (not structure) of the built output changes. So
 // eg, if we improve the linker's static analysis, this should be bumped.
-exports.BUILD_VERSION = 'meteor/1';
+exports.BUILD_VERSION = 'meteor/2';
 
 // Find all files under `rootPath` that have an extension in
 // `extensions` (an array of extensions without leading dot), and
@@ -1984,6 +1984,22 @@ _.extend(Package.prototype, {
       builder.reserve("head");
       builder.reserve("body");
 
+      // Pre-linker versions of Meteor expect all packages in the warehouse to
+      // contain a file called "package.js"; they use this as part of deciding
+      // whether or not they need to download a new package. Because packages
+      // are downloaded by the *existing* version of the tools, we need to
+      // include this file until we're comfortable breaking "meteor update" from
+      // 0.6.4.  (Specifically, warehouse.packageExistsInWarehouse used to check
+      // to see if package.js exists instead of just looking for the package
+      // directory.)
+      // XXX Remove this once we can.
+      builder.write("package.js", {
+        data: new Buffer(
+          ("// This file is included for compatibility with the Meteor " +
+           "0.6.4 package downloader.\n"),
+          "utf8")
+      };
+
       // Slices
       _.each(self.slices, function (slice) {
         if (! slice.isBuilt)
diff --git a/tools/warehouse.js b/tools/warehouse.js
index 4fc3eabcb8..91e47b3311 100644
--- a/tools/warehouse.js
+++ b/tools/warehouse.js
@@ -149,11 +149,12 @@ _.extend(warehouse, {
   },
 
   packageExistsInWarehouse: function (name, version) {
-    // Look for presence of "package.js" file in directory so we don't count
-    // an empty dir as a package.  An empty dir could be left by a failed
-    // package untarring, for example.
+    // A package exists if its directory exists. (We used to look for a
+    // particular file name ("package.js") inside the directory, but since we
+    // always install packages by untarring to a temporary directory and
+    // renaming atomically, we shouldn't worry about partial packages.)
     return fs.existsSync(
-      path.join(warehouse.getWarehouseDir(), 'packages', name, version, 'package.js'));
+      path.join(warehouse.getWarehouseDir(), 'packages', name, version));
   },
 
   getPackageFreshFile: function (name, version) {