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

runtime/vm/kernel_to_il.cc

 // 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.
 
 #include <set>
 
 #include "vm/kernel_to_il.h"
 
 #include "vm/compiler.h"
 #include "vm/intermediate_language.h"
 #include "vm/kernel_binary_flowgraph.h"
 #include "vm/kernel_reader.h"
 #include "vm/longjump.h"
 #include "vm/method_recognizer.h"
 #include "vm/object_store.h"
 #include "vm/report.h"
 #include "vm/resolver.h"
 #include "vm/stack_frame.h"
 
 #if !defined(DART_PRECOMPILED_RUNTIME)
 namespace dart {
 
-DECLARE_FLAG(bool, support_externalizable_strings);
-
 namespace kernel {
 
 #define Z (zone_)
 #define H (translation_helper_)
 #define T (type_translator_)
 #define I Isolate::Current()
 
 Fragment& Fragment::operator+=(const Fragment& other) {
   if (entry == NULL) {
     entry = other.entry;
@@ skipping 1635 matching lines @@
   Fragment body;
   MethodRecognizer::Kind kind = MethodRecognizer::RecognizeKind(function);
   switch (kind) {
     case MethodRecognizer::kObjectEquals:
       body += LoadLocal(scopes_->this_variable);
       body += LoadLocal(LookupVariable(first_positional_offset));
       body += StrictCompare(Token::kEQ_STRICT);
       break;
     case MethodRecognizer::kStringBaseLength:
     case MethodRecognizer::kStringBaseIsEmpty:
-      // Depending on FLAG_support_externalizable_strings, treat string length
-      // loads as mutable so that the class check that precedes them will not be
-      // hoisted.  This is unsafe because string externalization can change the
-      // class.
       body += LoadLocal(scopes_->this_variable);
       body += LoadNativeField(MethodRecognizer::kStringBaseLength,
                               dart::String::length_offset(),
                               Type::ZoneHandle(Z, Type::SmiType()), kSmiCid,
-                              !FLAG_support_externalizable_strings);
+                              /* is_immutable = */ true);
       if (kind == MethodRecognizer::kStringBaseIsEmpty) {
         body += IntConstant(0);
         body += StrictCompare(Token::kEQ_STRICT);
       }
       break;
     case MethodRecognizer::kGrowableArrayLength:
       body += LoadLocal(scopes_->this_variable);
       body += LoadNativeField(kind, GrowableObjectArray::length_offset(),
                               Type::ZoneHandle(Z, Type::SmiType()), kSmiCid);
       break;
@@ skipping 716 matching lines @@
   array_object = AsSortedDuplicateFreeArray(&token_positions);
   script.set_debug_positions(array_object);
   array_object = AsSortedDuplicateFreeArray(&yield_positions);
   script.set_yield_positions(array_object);
 }
 
 }  // namespace kernel
 }  // namespace dart
 
 #endif  // !defined(DART_PRECOMPILED_RUNTIME)