[build][dart] Enforce existence of *_test.dart files in dart_test

At least one *_test.dart must be passed to dart_test under |sources|.
While this was true in principle, it was not officially enforced.

Fixed: 39322
Change-Id: I73e3afeff339e8de7c9ee3d6fb180d0b114b31fd
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/383740
Reviewed-by: Isma Cortes <isma@google.com>
Testability-Review: Joshua Seaton <joshuaseaton@google.com>
Commit-Queue: Joshua Seaton <joshuaseaton@google.com>
diff --git a/build/dart/group_tests.py b/build/dart/group_tests.py
index dc91d30..b24e96a 100755
--- a/build/dart/group_tests.py
+++ b/build/dart/group_tests.py
@@ -29,6 +29,7 @@
     tests = filter(lambda src: src.endswith('_test.dart'), args.source)
+    assert len(tests) > 0, 'a least one *_test.dart file must be passed in under |sources|'
     imports = ["import 'package:test/test.dart';"]
     invocations = []
     for test in tests: