Add flag to disable string externalization

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.
 
 #include "vm/flow_graph_builder.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/ast_printer.h"
 #include "vm/bit_vector.h"
 #include "vm/compiler.h"
@@ skipping 22 matching lines @@
 namespace dart {
 
 DEFINE_FLAG(bool, eliminate_type_checks, 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 synthetic token position.
 #define ST(token_pos) ((token_pos).ToSynthetic())
 
 // TODO(srdjan): Allow compiler to add constants as they are encountered in
 // the compilation.
 const double kCommonDoubleConstants[] =
@@ skipping 3408 matching lines @@
       }
       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);
         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,
@@ skipping 1181 matching lines @@
                    Script::Handle(function.script()),
                    function.token_pos(),
                    Report::AtLocation,
                    "FlowGraphBuilder Bailout: %s %s",
                    String::Handle(function.name()).ToCString(),
                    reason);
   UNREACHABLE();
 }
 
 }  // namespace dart