Fixed Issue 7508: Many StringBuffer methods return StringBuffer, but should be void.

tests/co19/co19-runtime.status

 # 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.
 
 [ $compiler == none && $runtime == vm ]
 Language/13_Libraries_and_Scripts/1_Imports_A02_t21: Crash # Dart issue 6060
 Language/13_Libraries_and_Scripts/1_Imports_A02_t22: Crash # Dart issue 6060
 
 Language/03_Overview/1_Scoping_A02_t32: Fail # Dart issue 6556
 Language/06_Functions/2_Formal_Parameters/2_Optional_Formals_A01_t10: Fail # co19 issue 348
@@ skipping 284 matching lines @@
 
 
 [ $compiler == none && $runtime == vm ]
 Language/07_Classes/07_Classes_A02_t29: Fail
 Language/07_Classes/07_Classes_A02_t31: Fail
 Language/11_Expressions/05_Strings_A20_t01: Fail
 LibTest/isolate/ReceivePort/receive_A01_t02: Fail
 LibTest/isolate/isolate_api/spawnUri_A01_t03: Crash
 LibTest/isolate/isolate_api/spawnUri_A01_t04: Crash
 
+LibTest/core/StringBuffer/addAll_A02_t01:  Fail, OK # co19 issue 355
+LibTest/core/StringBuffer/add_A02_t01:  Fail, OK # co19 issue 355
+LibTest/core/StringBuffer/isEmpty_A01_t01:  Fail, OK # co19 issue 355
+LibTest/core/StringBuffer/clear_A02_t01: Fail, OK # co19 issue 355
 
 # parameter name or type expected
 Language/07_Classes/6_Constructors/1_Generative_Constructors_A15_t07: Fail
 
 
 [ $compiler == none && $runtime == vm && $checked ]
 Language/03_Overview/1_Scoping_A02_t30: Fail
 
 
 [ $compiler == none && $runtime == vm && $mode == debug ]
 LibTest/isolate/isolate_api/spawnFunction_A02_t01: Crash
 
 [ $compiler == none && $arch == simarm ]
 *: Skip
 
 [ $compiler == none && $arch == arm ]
 *: Skip