Add “pub cache repair” to forcibly re-install previously cached packages.

sdk/lib/_internal/pub/test/cache/repair/empty_cache_test.dart

Index: sdk/lib/_internal/pub/test/cache/repair/empty_cache_test.dart
diff --git a/sdk/lib/_internal/pub/test/cache/add/package_not_found_test.dart b/sdk/lib/_internal/pub/test/cache/repair/empty_cache_test.dart
similarity index 55%
copy from sdk/lib/_internal/pub/test/cache/add/package_not_found_test.dart
copy to sdk/lib/_internal/pub/test/cache/repair/empty_cache_test.dart
index 9e60b5f7790e88b3d365cc37e645e0d02c10310d..9f398b7635f3a481c460620429cfe73db3282b6e 100644
--- a/sdk/lib/_internal/pub/test/cache/add/package_not_found_test.dart
+++ b/sdk/lib/_internal/pub/test/cache/repair/empty_cache_test.dart
@@ -8,11 +8,9 @@ import '../../test_pub.dart';
 
 main() {
   initConfig();
-  integration('fails if the package cound not be found on the source', () {
-    servePackages([]);
-
-    schedulePub(args: ["cache", "add", "foo"],
-        error: new RegExp(r"Could not find package foo at http://.*"),
-        exitCode: 1);
+  integration('does nothing if the cache is empty', () {
+    // Repair them.
+    schedulePub(args: ["cache", "repair"],
+        output: "No packages in cache, so nothing to repair.");
   });
 }