Some fixes in unittest after io v2

pkg/unittest/lib/src/string_matchers.dart

 // Copyright (c) 2012, 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.
 
 part of matcher;
 
 /**
  * Returns a matcher which matches if the match argument is a string and
  * is equal to [value] when compared case-insensitively.
  */
@@ skipping 56 matching lines @@
  * and strip leading/trailing whitespace.
  */
 String collapseWhitespace(_string) {
   bool isWhitespace(String ch) => (' \n\r\t'.indexOf(ch) >= 0);
   StringBuffer result = new StringBuffer();
   bool skipSpace = true;
   for (var i = 0; i < _string.length; i++) {
     var character = _string[i];
     if (isWhitespace(character)) {
       if (!skipSpace) {
-        result.add(' ');
+        result.write(' ');
         skipSpace = true;
       }
     } else {
-      result.add(character);
+      result.write(character);
       skipSpace = false;
     }
   }
   return result.toString().trim();
 }
 
 /**
  * Returns a matcher that matches if the match argument is a string and
  * starts with [prefixString].
  */
@@ skipping 102 matching lines @@
     if (!(item is String)) {
       return mismatchDescription.
           addDescriptionOf(item).
           add(' not a string');
     } else {
       return super.describeMismatch(item, mismatchDescription,
           matchState, verbose);
     }
   }
 }