Throw a better error for async groups. (#486)

Closes #485

CHANGELOG.md

+## 0.12.15+10
+
+* Throw a better error if a group body is asynchronous.
+
 ## 0.12.15+9
 
 * Widen version constraint on `analyzer`.

lib/src/backend/declarer.dart

@@ -129,7 +129,13 @@ class Declarer {
 
     var declarer = new Declarer._(
         this, _prefix(name), metadata, _collectTraces, trace);
-    declarer.declare(body);
+    declarer.declare(() {
+      // Cast to dynamic to avoid the analyzer complaining about us using the
+      // result of a void method.
+      var result = (body as dynamic)();
+      if (result is! Future) return;
+      throw new ArgumentError("Groups may not be async.");
+    });
     _entries.add(declarer.build());
   }
 

pubspec.yaml

 name: test
-version: 0.12.15+9
+version: 0.12.15+10
 author: Dart Team <misc@dartlang.org>
 description: A library for writing dart unit tests.
 homepage: https://github.com/dart-lang/test

test/backend/declarer_test.dart

@@ -354,6 +354,12 @@ void main() {
           equals(new Duration(seconds: 15)));
     });
 
+    test("disallows asynchronous groups", () async {
+      declare(() {
+        expect(() => group("group", () async {}), throwsArgumentError);
+      });
+    });
+
     group(".setUp()", () {
       test("is scoped to the group", () async {
         var setUpRun = false;