Make a bunch of stuff in pub 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 1e26554cd72a177088c1a83779d6940b07e50b1d..e0d13457a399a07b9ff3c827c79571223e837366 100644
--- a/utils/pub/system_cache.dart
+++ b/utils/pub/system_cache.dart
@@ -84,7 +84,9 @@ class SystemCache {
   /// 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 ensureDir(tempDir).then((temp) {
+    // Make sure all errors propagate through future.
+    return defer(() {
+      var temp = ensureDir(tempDir);
       return io.createTempDir(join(temp, 'dir'));
     });
   }
@@ -92,8 +94,9 @@ class SystemCache {
   /// Delete's the system cache's internal temp directory.
   Future deleteTempDir() {
     log.fine('Clean up system cache temp directory $tempDir.');
-    return dirExists(tempDir).then((exists) {
-      if (!exists) return;
+    // Make sure all errors propagate through future.
+    return defer(() {
+      if (!dirExists(tempDir)) return;
       return deleteDir(tempDir);
     });
   }