Walk dynamic module graph synchronously.

I made this code asynchronous at first to accommodate the asynchronous
cache API, but walking the graph needs to happen synchronously so that
near-simultaneous dynamic import(...)s do not interleave.

packages/dynamic-import/client.js

@@ -18,56 +18,45 @@ Module.prototype.dynamicImport = function (id) {
       throw error;
     }
 
-    // Require the current module from the complete meta graph.
+    // Require the parent module from the complete meta graph.
     var meta = requireMeta(module.id);
-    var missingTree;
-    var localTree;
+    var versions = Object.create(null);
 
     function walk(meta) {
-      if (meta.dynamic) {
-        return meta.pending || (meta.pending = add(
-          meta.module.id,
-          meta.version
-        ).then(function () {
-          return allChildren(meta, walkChild);
-        }));
+      if (meta.dynamic && ! meta.pending) {
+        meta.pending = true;
+        versions[meta.module.id] = meta.version;
+        meta.eachChild(walkChild);
       }
     }
 
-    function add(id, version) {
-      return cache.check(id, version).then(function (code) {
-        addToTree(localTree = localTree || Object.create(null), id, code);
-      }, function (missing) {
-        addToTree(missingTree = missingTree || Object.create(null), id, 1);
-      });
-    }
-
     function walkChild(childModule) {
       return walk(childModule.exports);
     }
 
-    return allChildren(meta, walkChild, [id]).then(function () {
+    meta.eachChild(walkChild, [id]);
+
+    var localTree;
+    var missingTree;
+
+    return Promise.all(Object.keys(versions).map(function (id) {
+      return cache.check(id, versions[id]).then(function (code) {
+        addToTree(localTree = localTree || Object.create(null), id, code);
+      }, function (missing) {
+        addToTree(missingTree = missingTree || Object.create(null), id, 1);
+      });
+
+    })).then(function () {
       if (localTree) {
         installResults(localTree);
       }
+
       return missingTree && fetchMissing(missingTree);
+
     }).then(get);
   });
 };
 
-// The allChildren iteration includes all child modules imported by
-// meta.module, not just the child explicitly required here, so that
-// implicit modules like package.json will be included too.
-function allChildren(meta, callback, idsToRequire) {
-  var results = [];
-  // See meteor/packages/modules-runtime/meteor-install.js for the
-  // definition of meta.eachChild.
-  meta.eachChild(function (child) {
-    results.push(callback(child));
-  }, idsToRequire);
-  return Promise.all(results);
-}
-
 function fetchMissing(missingTree) {
   return new Promise(function (resolve, reject) {
     Meteor.call(