Make a bunch of stuff in pub synchronous.

utils/pub/source.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library source;
 
 import 'dart:async';
 import 'io.dart';
 import 'package.dart';
 import 'pubspec.dart';
 import 'system_cache.dart';
+import 'utils.dart';
 import 'version.dart';
 
 /// A source from which to install packages.
 ///
 /// Each source has many packages that it looks up using [PackageId]s. The
 /// source is responsible for installing these packages to the package cache.
 abstract class Source {
   /// The name of the source. Should be lower-case, suitable for use in a
   /// filename, and unique accross all sources.
   String get name;
@@ skipping 76 matching lines @@
     throw "Either install or installToSystemCache must be implemented for "
         "source $name.";
   }
 
   /// Installs the package identified by [id] to the system cache. This is only
   /// called for sources with [shouldCache] set to true.
   ///
   /// By default, this uses [systemCacheDirectory] and [install].
   Future<Package> installToSystemCache(PackageId id) {
     var path = systemCacheDirectory(id);
-    return exists(path).then((exists) {
-      if (exists) return new Future<bool>.immediate(true);
-      return ensureDir(dirname(path)).then((_) => install(id, path));
+    // Make sure all errors propagate through future.
+    return defer(() {
+      if (entryExists(path)) return true;
+      ensureDir(dirname(path));
+      return install(id, path);
     }).then((found) {
       if (!found) throw 'Package $id not found.';
-      return Package.load(id.name, path, systemCache.sources);
+      return new Package(id.name, path, systemCache.sources);
     });
   }
 
   /// Returns the directory in the system cache that the package identified by
   /// [id] should be installed to. This should return a path to a subdirectory
   /// of [systemCacheRoot].
   ///
   /// This doesn't need to be implemented if [shouldCache] is false, or if
   /// [installToSystemCache] is implemented.
   String systemCacheDirectory(PackageId id) {
@@ skipping 37 matching lines @@
   /// according to [validateDescription], although it must still be serializable
   /// to JSON and YAML. It must also be equal to [id] according to
   /// [descriptionsEqual].
   ///
   /// By default, this just returns [id].
   Future<PackageId> resolveId(PackageId id) => new Future.immediate(id);
 
   /// Returns the source's name.
   String toString() => name;
 }