Don't allow packages to depend on themselves.

sdk/lib/_internal/pub/test/pubspec_test.dart

Index: sdk/lib/_internal/pub/test/pubspec_test.dart
diff --git a/sdk/lib/_internal/pub/test/pubspec_test.dart b/sdk/lib/_internal/pub/test/pubspec_test.dart
index 47f838a0a7f2fb895a1e9e4f8ac3ce7e6c6748bf..ddc5538e6b11553b317c922f3f1c157ce718d1eb 100644
--- a/sdk/lib/_internal/pub/test/pubspec_test.dart
+++ b/sdk/lib/_internal/pub/test/pubspec_test.dart
@@ -103,6 +103,24 @@ dev_dependencies:
 ''');
     });
 
+    test("throws if it dependes on itself", () {
+      expectFormatError('''
+name: myapp
+dependencies:
+  myapp:
+    mock: ok
+''');
+    });
+
+    test("throws if it has a dev dependency on itself", () {
+      expectFormatError('''
+name: myapp
+dev_dependencies:
+  myapp:
+    mock: ok
+''');
+    });
+
     test("throws if the description isn't valid", () {
       expectFormatError('''
 dependencies: