[vm] Remove Dart_MakeExternalString and --support-externalizable-strings

runtime/tests/vm/vm.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.
 
 # Issue 28198
 cc/IsolateReload_PendingUnqualifiedCall_InstanceToStatic: SkipSlow
 cc/IsolateReload_PendingUnqualifiedCall_StaticToInstance: SkipSlow
 cc/IsolateReload_PendingConstructorCall_AbstractToConcrete: SkipSlow
 cc/IsolateReload_PendingConstructorCall_ConcreteToAbstract: SkipSlow
 cc/IsolateReload_PendingStaticCall_DefinedToNSM: SkipSlow
@@ skipping 28 matching lines @@
 # It can take some time for all the isolates to shutdown in a Debug build.
 dart/spawn_shutdown_test: Pass, Slow  # VM Shutdown test
 
 # CodeImmutability test ocassionally times out on Windows. Issue #30202.
 [ $system != windows ]
 cc/CodeImmutability: Crash
 
 [ $system == windows ]
 cc/CodeImmutability: Crash, Timeout
 cc/Dart2JSCompileAll: Skip
-cc/ExternalizeConstantStrings: Skip
 cc/Service_Profile: Skip
 cc/Dart2JSCompilerStats: Skip
 cc/CorelibCompilerStats: Skip
 
 [ $system == fuchsia ]
 # OOM crash can bring down the OS.
 dart/spawn_shutdown_test: Skip
 cc/CorelibIsolateStartup: Skip
 # TODO(zra): package:unittest is not in the image.
 dart/data_uri_spawn_test: Skip
@@ skipping 381 matching lines @@
 cc/IsolateReload_TearOff_Library_Identity: Crash
 cc/IsolateReload_TearOff_List_Set: Crash
 cc/IsolateReload_TypeIdentity: Crash
 cc/IsolateReload_TypeIdentityGeneric: Crash
 cc/IsolateReload_TypeIdentityParameter: Crash
 cc/Parser_AllocateVariables_CaptureLoopVar: Crash
 cc/Parser_AllocateVariables_Issue7681: Crash
 cc/Parser_AllocateVariables_MiddleChain: Crash
 cc/Parser_AllocateVariables_NestedCapturedVar: Crash
 cc/Parser_AllocateVariables_TwoChains: Crash