Add some support for the code-point code-unit distinction.

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.
 
 # These tests either wrongly expect a compile-time error or wrongly reject a type error in production mode, co19 issue 282:
 [ $compiler == none && $runtime == vm ]
 Language/13_Libraries_and_Scripts/1_Imports_A03_t05: Fail, OK
 Language/13_Libraries_and_Scripts/1_Imports_A03_t08: Fail, OK
 Language/13_Libraries_and_Scripts/1_Imports_A03_t25: Fail, OK
 Language/13_Libraries_and_Scripts/1_Imports_A03_t28: Fail, OK
@@ skipping 181 matching lines @@
 LibTest/isolate/isolate_api/spawnUri_A01_t04: Fail # TODO(vm-team): Please triage this failure.
 LibTest/math/Random/nextDouble_A01_t01: Fail # TODO(vm-team): Please triage this failure.
 LibTest/math/parseDouble_A02_t01: Fail # TODO(vm-team): Please triage this failure.
 LibTest/math/pow_A01_t01: Fail # TODO(vm-team): Please triage this failure.
 LibTest/math/pow_A11_t01: Fail # TODO(vm-team): Please triage this failure.
 LibTest/math/pow_A13_t01: Fail # TODO(vm-team): Please triage this failure.
 LibTest/math/sin_A01_t01: Fail # TODO(vm-team): Please triage this failure.
 
 LibTest/isolate/isolate_api/port_A01_t01: Skip # Times out.
 
-LibTest/core/String/String_class_A02_t01: Fail, OK # co19 issue 284
-LibTest/core/String/charCodeAt_A01_t01: Fail, OK # co19 issue 284
-LibTest/core/String/charCodes_A01_t01: Fail, OK # co19 issue 284
-
 Language/06_Functions/1_Function_Declaration_A02_t03: Fail # issue 6058
 Language/06_Functions/1_Function_Declaration_A03_t03: Fail # issue 6058
 Language/06_Functions/2_Formal_Parameters/2_Optional_Formals_A03_t01: Fail # issue 6085
 Language/06_Functions/2_Formal_Parameters/2_Optional_Formals_A03_t02: Fail # issue 6085
 Language/06_Functions/2_Formal_Parameters/2_Optional_Formals_A03_t03: Fail # issue 6085
 
 LibTest/core/EmptyQueueException/EmptyQueueException_A01_t01: Fail, OK # co19 issue 288
 LibTest/core/EmptyQueueException/toString_A01_t01: Fail, OK # co19 issue 288
 LibTest/core/List/iterator_next_A02_t01: Fail, OK # co19 issue 288
 LibTest/core/NoMoreElementsException/NoMoreElementsException_A01_t01: Fail, OK # co19 issue 288
@@ skipping 208 matching lines @@
 
 
 [ $compiler == none && $runtime == vm && $mode == debug ]
 LibTest/isolate/isolate_api/spawnFunction_A02_t01: Crash
 
 [ $compiler == none && $arch == simarm ]
 *: Skip
 
 [ $compiler == none && $arch == arm ]
 *: Skip