Improve the event handling for string input stream

tests/standalone/src/StringStreamTest.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class ListInputStream implements InputStream {
   List<int> read() {
     var result = _data;
     _data = null;
     return result;
   }
@@ skipping 99 matching lines @@
     if (stage == 0) {
       line = stream.readLine();
       Expect.equals(null, line);
       stage++;
       s.close();
     } else if (stage == 1) {
       line = stream.readLine();
       Expect.equals("Line", line);
       line = stream.readLine();
       Expect.equals(null, line);
-      Expect.equals(true, stream.closed);
       stage++;
     }
   }
 
   void streamClosed() {
+    Expect.equals(true, stream.closed);
     Expect.equals(2, stage);
   }
 
   stream.dataHandler = stringData;
   stream.closeHandler = streamClosed;
   s.write("Line".charCodes());
-  Expect.equals(2, stage);
 }
 
 void testReadLine2() {
   InputStream s = new ListInputStream();
   StringInputStream stream = new StringInputStream(s);
   var stage = 0;
 
   void stringData() {
     var line;
     if (stage == 0) {
@@ skipping 24 matching lines @@
       line = stream.readLine();
       Expect.equals(null, line);
       stage++;
       s.close();
     } else if (stage == 3) {
       // The final \r can now be interpreted as an end of line.
       line = stream.readLine();
       Expect.equals("", line);
       line = stream.readLine();
       Expect.equals(null, line);
-      Expect.equals(true, stream.closed);
       stage++;
     }
   }
 
   void streamClosed() {
     Expect.equals(4, stage);
+    Expect.equals(true, stream.closed);
   }
 
   stream.lineHandler = stringData;
   stream.closeHandler = streamClosed;
   s.write("Line1\nLine2\r\nLine3\rLi".charCodes());
-  Expect.equals(4, stage);
 }
 
 main() {
   testUtf8();
   testLatin1();
   testAscii();
   testReadLine1();
   testReadLine2();
 }