Fix some corelib tests.

tests/corelib/collection_to_string_test.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.
 
 /**
  * Tests for the toString methods on collections and maps.
  */
 
 library collection_to_string;
 
@@ skipping 223 matching lines @@
     // It's an iterable, it may omit some elements.
     int end = stringRep.length;
     if (size > MAX_COUNT) {
       // Last two elements are also omitted, just find the first three or
       // first 60 characters.
       for (int i = MIN_COUNT; i < size; i++) {
         int startIndex = indices[i];
         if (startIndex - start > MAX_LENGTH - 6) {  // Limit - ", ...)".length.
           String prefix = stringRep.toString().substring(0, startIndex);
           stringRep.clear();
-          stringRep.add(prefix);
-          stringRep.add(", ...");
+          stringRep.write(prefix);
+          stringRep.write(", ...");
         }
       }
     } else if (stringRep.length - start > MAX_LENGTH - 1) {  // 80 - ")".length.
       // Last two elements are always included. Middle ones may be omitted.
       int lastTwoLength = end - indices[indices.length - 2];
       // Try to find first element to omit.
       for (int i = 3; i <= size - 3; i++) {
         int elementEnd = indices[i + 1];
         int lengthAfter = elementEnd - start;
         int ellipsisSize = 5;  // ", ...".length
@@ skipping 100 matching lines @@
   return rand.nextBool();
 }
 
 /** Returns the alphabetized characters in a string. */
 String alphagram(String s) {
   // Calling [toList] to convert unmodifiable list to normal list.
   List<int> chars = s.codeUnits.toList();
   chars.sort((int a, int b) => a - b);
   return new String.fromCharCodes(chars);
 }