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

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

Index: sdk/lib/_internal/pub/lib/src/pubspec.dart
diff --git a/sdk/lib/_internal/pub/lib/src/pubspec.dart b/sdk/lib/_internal/pub/lib/src/pubspec.dart
index 55b2df7ba57935ac2744e369b76012169e6515e1..6781a5c1df07afb8a298c3c1ea921cda03fabeff 100644
--- a/sdk/lib/_internal/pub/lib/src/pubspec.dart
+++ b/sdk/lib/_internal/pub/lib/src/pubspec.dart
@@ -74,6 +74,14 @@ class Pubspec {
     }
 
     var span = fields.nodes['version'].span;
+    if (version is num) {
+      var fixed = '$version.0';
+      if (version is int) {
+        fixed = '$fixed.0';
+      }
+      _error('"version" field must have three numeric components: major, '
+          'minor, and patch. Instead of "$version", consider "$fixed".', span);
+    }
     if (version is! String) {
       _error('"version" field must be a string.', span);
     }