Move StringBufferImpl from coreimpl to core as _StringBufferImpl.

tests/language/language.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.
 
 # This directory contains tests that are intended to show the
 # current state of the language.
 
 # In order to maintain maximum test coverage for all builds,
 # please use the following procedure to mark a test
 # failed on architectures other than the one you are working on.
 #
 # 1) Copy the old version of the test to
 #    tests/language/src/test_name_[dartc|vm]_test.dart.
 #    to maintain coverage.
 # 2) File a bug on each architecture for the failure due to the language change.
 # 3) Update the language/src directory with the updated test.
 
 [ $compiler == none ]
 part_test: Fail
 part2_test: Fail
-gc_test: Fail # Issue 1487
 field_override_test/none: Fail  # Issue 742: field shadowing now allowed
 field_override_test/01: Fail    # Issue 742: field shadowing now allowed
 super_field_access_test: Fail    # Issue 742: field shadowing now allowed
 pseudo_kw_illegal_test/03: Fail  # Issue 356
 pseudo_kw_illegal_test/08: Fail  # Issue 356
 pseudo_kw_illegal_test/10: Fail  # Issue 356
 pseudo_kw_illegal_test/14: Fail  # Issue 356
 
 # These bugs refer currently ongoing language discussions.
 constructor5_test: Fail          # (Discussion ongoing)
@@ skipping 397 matching lines @@
 compile_time_constant_checked3_test/04: Fail, OK
 compile_time_constant_checked3_test/05: Fail, OK
 compile_time_constant_checked3_test/06: Fail, OK
 
 type_error_test: Fail, OK # VM bug: http://dartbug.com/5280
 
 call_through_getter_test: Fail # issue 6130
 call_through_null_getter_test: Fail # issue 6130
 local_function_test: Fail # issue 6130
 naming_test: Fail # issue 6130