pkg/unittest: config refactor

pkg/unittest/lib/unittest.dart

Index: pkg/unittest/lib/unittest.dart
diff --git a/pkg/unittest/lib/unittest.dart b/pkg/unittest/lib/unittest.dart
index 0d9a549b727c8603f17a8377dabdbed44cfe4637..9bb75808ad944a9e127b077ed8d089c5ca4146d0 100644
--- a/pkg/unittest/lib/unittest.dart
+++ b/pkg/unittest/lib/unittest.dart
@@ -156,6 +156,7 @@ library unittest;
 import 'dart:async';
 import 'dart:isolate';
 import 'matcher.dart';
+import 'package:meta/meta.dart';
 export 'matcher.dart';
 
 // TODO(amouravski): We should not need to import mock here, but it's necessary
@@ -165,20 +166,33 @@ import 'mock.dart';
 part 'src/config.dart';
 part 'src/test_case.dart';
 
-/** [Configuration] used by the unittest library. */
-Configuration _config = null;
+Configuration _config;
 
+/** [Configuration] used by the unittest library. */
 Configuration get config => _config;
 
 /**
- * Set the [Configuration] used by the unittest library. Returns any
- * previous configuration.
- * TODO: consider deprecating in favor of a setter now we have a getter.
+ * Sets the [Configuration] used by the unittest library.
+ *
+ * Throws an exception if a value has already been set to a different value.
+ */
+void set config(Configuration value) {
+  if(!identical(_config, value)) {
+    if(_config != null) {
+      throw 'config has already been set';
+    }
+    _config = value;
+  }
+}
+
+/**
+ * Use [config] instead.
  */
-Configuration configure(Configuration config) {
-  Configuration _oldConfig = _config;
-  _config = config;
-  return _oldConfig;
+@deprecated
+Configuration configure(Configuration value) {
+  var oldConfig = config;
+  config = value;
+  return oldConfig;
 }
 
 void logMessage(String message) => _config.logMessage(message);
@@ -807,7 +821,7 @@ void ensureInitialized() {
   _uncaughtErrorMessage = null;
 
   if (_config == null) {
-    _config = new Configuration();
+    config = new Configuration();
   }
   _config.onInit();