Printing a useful error message if a version field is a num

sdk/lib/_internal/pub_generated/lib/src/pubspec.dart

Index: sdk/lib/_internal/pub_generated/lib/src/pubspec.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/pubspec.dart b/sdk/lib/_internal/pub_generated/lib/src/pubspec.dart
index b8b28ed154b137d4d85339a71536410cea382528..a997ee07a3a4002bf17b5a3286756dfb72160b58 100644
--- a/sdk/lib/_internal/pub_generated/lib/src/pubspec.dart
+++ b/sdk/lib/_internal/pub_generated/lib/src/pubspec.dart
@@ -37,6 +37,9 @@ class Pubspec {
       return _version;
     }
     var span = fields.nodes['version'].span;
+    if (version is num) {
+      _error('"version" field must be written in MAJOR.MINOR.PATCH[SUFFIX] format', span);
+    }
     if (version is! String) {
       _error('"version" field must be a string.', span);
     }