Added isClosed, addresses simc/logger#130

Author: Bungeefan

lib/src/logger.dart

@@ -142,6 +142,10 @@ class Logger {
     }
   }
 
+  bool isClosed() {
+    return !_active;
+  }
+
   /// Closes the logger and releases all resources.
   void close() {
     _active = false;

test/logger_test.dart

@@ -168,4 +168,11 @@ void main() {
     logger.w('This is');
     expect(printedMessage, 'This is');
   });
+
+  test('Logger.close', () {
+    var logger = Logger();
+    expect(logger.isClosed(), false);
+    logger.close();
+    expect(logger.isClosed(), true);
+  });
 }