Fix String.trim implementation (follow spec).

runtime/lib/string_buffer.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.
 
 /**
  * The StringBuffer class is useful for concatenating strings
  * efficiently. Only on a call to [toString] are the strings
  * concatenated to a single String.
  */
 class StringBufferImpl implements StringBuffer {
@@ skipping 14 matching lines @@
 
   bool isEmpty() {
     return _length === 0;
   }
 
   /**
    * Adds [obj] to the buffer. Returns [this].
    */
   StringBuffer add(Object obj) {
     String str = obj.toString();
-    if (str === null || str.isEmpty()) return;
+    if (str === null || str.isEmpty()) {
+      return this;
+    }
     _buffer.add(str);
     _length += str.length;
     return this;
   }
 
   /**
    * Adds all items in [objects] to the buffer. Returns [this].
    */
   StringBuffer addAll(Collection<Object> objects) {
     for (Object obj in objects) {
@@ skipping 29 matching lines @@
     _buffer.clear();
     _buffer.add(result);
     // Since we track the length at each add operation, there is no
     // need to update it in this function.
     return result;
   }
 
   List<String> _buffer;
   int _length;
 }