Use more semantic exit codes so we can detect HTTP errors.

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 1a1534752ce0ce71a0e63e3fe7c10cc3b935e513..1d9a8477ce0b8baf93d4f33467d9f02345cba897 100644
--- a/utils/pub/hosted_source.dart
+++ b/utils/pub/hosted_source.dart
@@ -39,7 +39,7 @@ class HostedSource extends Source {
       var doc = JSON.parse(body);
       return doc['versions'].map((version) => new Version.parse(version));
     }).transformException((ex) {
-      if (ex is HttpException && ex.statusCode == 404) {
+      if (ex is PubHttpException && ex.statusCode == 404) {
         throw 'Could not find package "${parsed.first}" on ${parsed.last}.';
       }