Make createTempDir() synchronous.

utils/pub/system_cache.dart

Index: utils/pub/system_cache.dart
diff --git a/utils/pub/system_cache.dart b/utils/pub/system_cache.dart
index 1b5d9820b79ad20a7d43c410d63277511e833ca7..13e6b8d68db8d6ca2119339b00abd2c61cb32fc1 100644
--- a/utils/pub/system_cache.dart
+++ b/utils/pub/system_cache.dart
@@ -101,11 +101,9 @@ class SystemCache {
   /// packages into while installing. It uses this instead of the OS's system
   /// temp directory to ensure that it's on the same volume as the pub system
   /// cache so that it can move the directory from it.
-  Future<Directory> createTempDir() {
-    return defer(() {
-      var temp = ensureDir(tempDir);
-      return io.createTempDir(join(temp, 'dir'));
-    });
+  Directory createTempDir() {
+    var temp = ensureDir(tempDir);
+    return io.createTempDir(join(temp, 'dir'));
   }
 
   /// Delete's the system cache's internal temp directory.