Download packages to temp dir inside system cache.

utils/pub/hosted_source.dart

Index: utils/pub/hosted_source.dart
diff --git a/utils/pub/hosted_source.dart b/utils/pub/hosted_source.dart
index 2d5c1f5900752fb5eb7b6354223c7e236d5b46d0..4ca407420b45dc165e8bc40f6840f568ee0837f6 100644
--- a/utils/pub/hosted_source.dart
+++ b/utils/pub/hosted_source.dart
@@ -77,7 +77,8 @@ class HostedSource extends Source {
 
     // Download and extract the archive to a temp directory.
     var tempDir;
-    return Futures.wait([httpGet(fullUrl), createTempDir()]).chain((args) {
+    return Futures.wait([httpGet(fullUrl),
+                         systemCache.createTempDir()]).chain((args) {
       tempDir = args[1];
       return timeout(extractTarGz(args[0], tempDir), HTTP_TIMEOUT,
           'Timed out while fetching URL "$fullUrl".');