Fix a bug in pubspec parsing.

lib/src/pubspec.dart

Index: lib/src/pubspec.dart
diff --git a/lib/src/pubspec.dart b/lib/src/pubspec.dart
index 19bd769fcef94b3cfe96bb9cd7ff758996080cab..28b1375cbf9e07ad52445b89d8e7c9c9f300db1a 100644
--- a/lib/src/pubspec.dart
+++ b/lib/src/pubspec.dart
@@ -463,6 +463,8 @@ class Pubspec {
         var sourceNames = spec.keys.toList();
         if (sourceNames.length > 1) {
           _error('A dependency may only have one source.', specNode.span);
+        } else if (sourceNames.isEmpty) {
+          _error('A dependency must contain a source.', specNode.span);
         }
 
         sourceName = sourceNames.single;