Fix analyzer errors/warnings for Pub.

utils/pub/source.dart

Index: utils/pub/source.dart
diff --git a/utils/pub/source.dart b/utils/pub/source.dart
index b08647555e4556efae93e041f859de6513dabab4..1fcf9c87a5c140505559f4d50f83fa744a18fdc5 100644
--- a/utils/pub/source.dart
+++ b/utils/pub/source.dart
@@ -143,8 +143,10 @@ abstract class Source {
    * This doesn't need to be implemented if [shouldCache] is false, or if
    * [installToSystemCache] is implemented.
    */
-  String systemCacheDirectory(PackageId id) =>
-    join(systemCacheRoot, packageName(id.description));
+  String systemCacheDirectory(PackageId id) {
+    throw 'Source.systemCacheDirectory must be implemented if shouldCache is '
+        'true and installToSystemCache is not implemented.';
+  }
 
   /**
    * When a [Pubspec] or [LockFile] is parsed, it reads in the description for