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

runtime/vm/flow_graph_builder.cc

 // 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.
 
 #if !defined(DART_PRECOMPILED_RUNTIME)
 
 #include "vm/flow_graph_builder.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/ast_printer.h"
@@ skipping 28 matching lines @@
             true,
             "Eliminate type checks when allowed by static type analysis.");
 DEFINE_FLAG(bool, print_ast, false, "Print abstract syntax tree.");
 DEFINE_FLAG(bool, print_scopes, false, "Print scopes of local variables.");
 DEFINE_FLAG(bool,
             trace_type_check_elimination,
             false,
             "Trace type check elimination at compile time.");
 
 DECLARE_FLAG(bool, profile_vm);
-DECLARE_FLAG(bool, support_externalizable_strings);
 
 // Quick access to the locally defined zone() method.
 #define Z (zone())
 
 // Quick access to the locally defined thread() method.
 #define T (thread())
 
 // Quick synthetic token position.
 #define ST(token_pos) ((token_pos).ToSynthetic())
 
@@ skipping 3111 matching lines @@
         Value* other = Bind(new (Z) LoadLocalInstr(*other_var, token_pos));
         // Receiver is not a number because numbers override equality.
         const bool kNoNumberCheck = false;
         StrictCompareInstr* compare = new (Z)
             StrictCompareInstr(token_pos, Token::kEQ_STRICT, receiver, other,
                                kNoNumberCheck, owner()->GetNextDeoptId());
         return ReturnDefinition(compare);
       }
       case MethodRecognizer::kStringBaseLength:
       case MethodRecognizer::kStringBaseIsEmpty: {
-        // Treat length loads as mutable (i.e. affected by side effects) to
-        // avoid hoisting them since we can't hoist the preceding class-check.
-        // This is because of externalization of strings that affects their
-        // class-id.
         LoadFieldInstr* load = BuildNativeGetter(
             node, MethodRecognizer::kStringBaseLength, String::length_offset(),
             Type::ZoneHandle(Z, Type::SmiType()), kSmiCid);
-        load->set_is_immutable(!FLAG_support_externalizable_strings);
+        load->set_is_immutable(true);
         if (kind == MethodRecognizer::kStringBaseLength) {
           return ReturnDefinition(load);
         }
         ASSERT(kind == MethodRecognizer::kStringBaseIsEmpty);
         Value* zero_val =
             Bind(new (Z) ConstantInstr(Smi::ZoneHandle(Z, Smi::New(0))));
         Value* load_val = Bind(load);
         StrictCompareInstr* compare = new (Z) StrictCompareInstr(
             token_pos, Token::kEQ_STRICT, load_val, zero_val, false,
             owner()->GetNextDeoptId());  // No number check.
@@ skipping 1218 matching lines @@
   // Bail if the type has any type parameters.
   if (type_class.IsGeneric()) return false;
 
   // Finally a simple class for instance of checking.
   return true;
 }
 
 }  // namespace dart
 
 #endif  // !defined(DART_PRECOMPILED_RUNTIME)