VM: [Kernel] Add kernel::ConstantEvaluator support for string.length

tests/co19/co19-kernel.status

 # Copyright (c) 2016, 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.
 
 # Disable tests globally for kernel.
 [ $compiler == dartk || $compiler == dartkp ]
 Language/Libraries_and_Scripts/Imports/static_type_t01: Skip # No support for deferred libraries.
 Language/Metadata/*: Skip # No support for metadata ATM.
 LibTest/async/DeferredLibrary/DeferredLibrary_A01_t01: Skip # No support for deferred libraries.
 LibTest/isolate/*: Skip # No support for isolates ATM.
@@ skipping 108 matching lines @@
 ###############################################################################
 # VM Entries
 ###############################################################################
 
 # dartk: Shared JIT & Precompiled failures
 [ ($compiler == dartk && $runtime == vm) || ($compiler == dartkp && $runtime == dart_precompiled) ]
 Language/Classes/Constructors/Generative_Constructors/execution_of_a_superinitializer_t01: RuntimeError
 Language/Classes/Constructors/Generative_Constructors/execution_of_an_initializer_t02: RuntimeError
 Language/Classes/Constructors/Generative_Constructors/initializing_formals_execution_t02: RuntimeError
 Language/Classes/definition_t23: CompileTimeError
-Language/Expressions/Constants/string_length_t01: Crash
 Language/Expressions/Function_Invocation/Function_Expression_Invocation/not_a_function_expression_t01: RuntimeError
 Language/Expressions/Function_Invocation/Unqualified_Invocation/function_expr_invocation_t03: RuntimeError
 Language/Expressions/Function_Invocation/Unqualified_Invocation/function_expr_invocation_t04: RuntimeError
 Language/Expressions/Function_Invocation/Unqualified_Invocation/static_method_invocation_t02: RuntimeError
 Language/Expressions/Identifier_Reference/built_in_identifier_t35: Pass
 Language/Expressions/Identifier_Reference/built_in_identifier_t36: Pass
 Language/Expressions/Identifier_Reference/built_in_identifier_t37: Pass
 Language/Expressions/Identifier_Reference/built_in_not_dynamic_t14: Pass
 Language/Expressions/Identifier_Reference/evaluation_property_extraction_t03: RuntimeError
 Language/Expressions/Instance_Creation/New/evaluation_t12: RuntimeError
@@ skipping 50 matching lines @@
 # dartk: precompilation failures
 [ $compiler == dartkp && $runtime == dart_precompiled ]
 LibTest/core/StringBuffer/StringBuffer_A01_t02: RuntimeError
 LibTest/core/StringBuffer/write_A01_t02: RuntimeError
 Language/Expressions/Constants/exception_t01: Crash
 Language/Expressions/Constants/exception_t02: Crash
 
 # dartk: precompilation failures (debug)
 [ $compiler == dartkp && $runtime == dart_precompiled && $mode == debug ]
 Language/Libraries_and_Scripts/Scripts/top_level_main_t05: Crash  # !main_obj.IsNull()