Make a bunch of stuff in pub synchronous.

utils/pub/source.dart

Index: utils/pub/source.dart
diff --git a/utils/pub/source.dart b/utils/pub/source.dart
index 211f0dbad4dc342350d9714ee83e5d18d38858f0..0db778299506c2b813b7c785f84a4fe737fa6e48 100644
--- a/utils/pub/source.dart
+++ b/utils/pub/source.dart
@@ -9,6 +9,7 @@ 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.
@@ -105,12 +106,14 @@ abstract class Source {
   /// 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);
     });
   }