Add stack chain support to pub.

sdk/lib/_internal/pub/lib/src/source.dart

Index: sdk/lib/_internal/pub/lib/src/source.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source.dart b/sdk/lib/_internal/pub/lib/src/source.dart
index a27a9095442cd9c116befc32d424f89bf9e8da35..299f6cfc75eb97f42e1c5ccf142e7132ac87c82d 100644
--- a/sdk/lib/_internal/pub/lib/src/source.dart
+++ b/sdk/lib/_internal/pub/lib/src/source.dart
@@ -7,6 +7,7 @@ library pub.source;
 import 'dart:async';
 
 import 'package:path/path.dart' as path;
+import 'package:stack_trace/stack_trace.dart';
 
 import 'io.dart';
 import 'package.dart';
@@ -203,7 +204,8 @@ abstract class Source {
   /// This doesn't need to be implemented if [shouldCache] is false.
   Future<String> systemCacheDirectory(PackageId id) {
     return new Future.error(
-        "systemCacheDirectory() must be implemented if shouldCache is true.");
+        "systemCacheDirectory() must be implemented if shouldCache is true.",
+        new Chain.current());
   }
 
   /// When a [Pubspec] or [LockFile] is parsed, it reads in the description for