feature: tag tests; choose tags on command line

lib/test.dart

Index: lib/test.dart
diff --git a/lib/test.dart b/lib/test.dart
index 4d19aa0151475da939b017571da2fa00fb1af768..c3c68d6e9594913ef1897b9e756263bac09f6e27 100644
--- a/lib/test.dart
+++ b/lib/test.dart
@@ -25,6 +25,7 @@ export 'src/frontend/future_matchers.dart';
 export 'src/frontend/on_platform.dart';
 export 'src/frontend/prints_matcher.dart';
 export 'src/frontend/skip.dart';
+export 'src/frontend/tags.dart';
 export 'src/frontend/test_on.dart';
 export 'src/frontend/throws_matcher.dart';
 export 'src/frontend/throws_matchers.dart';
@@ -112,11 +113,18 @@ Declarer get _declarer {
 ///
 /// If multiple platforms match, the annotations apply in order as through
 /// they were in nested groups.
-void test(String description, body(), {String testOn, Timeout timeout,
-        skip, Map<String, dynamic> onPlatform, String tag,
-        List<String> tags}) => _declarer.test(description, body,
-            testOn: testOn, timeout: timeout, skip: skip,
-            onPlatform: onPlatform, tags: _deconvenienceTags(tag, tags));
+void test(String description, body(),
+        {String testOn,
+        Timeout timeout,
+        skip,
+        Map<String, dynamic> onPlatform,
+        tags}) =>
+    _declarer.test(description, body,
+        testOn: testOn,
+        timeout: timeout,
+        skip: skip,
+        onPlatform: onPlatform,
+        tags: tags);
 
 /// Creates a group of tests.
 ///
@@ -157,11 +165,14 @@ void test(String description, body(), {String testOn, Timeout timeout,
 ///
 /// If multiple platforms match, the annotations apply in order as through
 /// they were in nested groups.
-void group(String description, void body(), {String testOn, Timeout timeout,
-        skip, Map<String, dynamic> onPlatform, String tag,
-        List<String> tags}) => _declarer.group(description, body,
-            testOn: testOn, timeout: timeout, skip: skip,
-            tags: _deconvenienceTags(tag, tags));
+void group(String description, void body(),
+        {String testOn,
+        Timeout timeout,
+        skip,
+        Map<String, dynamic> onPlatform,
+        tags}) =>
+    _declarer.group(description, body,
+        testOn: testOn, timeout: timeout, skip: skip, tags: tags);
 
 /// Registers a function to be run before tests.
 ///
@@ -223,13 +234,3 @@ void registerException(error, [StackTrace stackTrace]) {
   // going through the zone API allows other zones to consistently see errors.
   Zone.current.handleUncaughtError(error, stackTrace);
 }
-
-List<String> _deconvenienceTags(String tag, List<String> tags) {
-  var result = const[];
-  if (tag != null || (tags != null && tags.isNotEmpty)) {
-    result = [];
-    if (tag != null) result.add(tag);
-    if (tags != null && tags.isNotEmpty) result.addAll(tags);
-  }
-  return result;
-}