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

runtime/vm/flow_graph_inliner.cc

Index: runtime/vm/flow_graph_inliner.cc
diff --git a/runtime/vm/flow_graph_inliner.cc b/runtime/vm/flow_graph_inliner.cc
index 8e10ce1e082e5346f70597d8ff9dc740a4a57071..b8f8fb4327f4c0b02ce5b81f41ad8d81e2792baa 100644
--- a/runtime/vm/flow_graph_inliner.cc
+++ b/runtime/vm/flow_graph_inliner.cc
@@ -98,7 +98,6 @@ DECLARE_FLAG(bool, compiler_stats);
 DECLARE_FLAG(int, max_deoptimization_counter_threshold);
 DECLARE_FLAG(bool, print_flow_graph);
 DECLARE_FLAG(bool, print_flow_graph_optimized);
-DECLARE_FLAG(bool, support_externalizable_strings);
 DECLARE_FLAG(bool, verify_compiler);
 
 // Quick access to the current zone.
@@ -2730,15 +2729,11 @@ static Definition* PrepareInlineStringIndexOp(FlowGraph* flow_graph,
                                               Definition* str,
                                               Definition* index,
                                               Instruction* cursor) {
-  // Load the length of the string.
-  // 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* length = new (Z)
       LoadFieldInstr(new (Z) Value(str), String::length_offset(),
                      Type::ZoneHandle(Z, Type::SmiType()), str->token_pos());
   length->set_result_cid(kSmiCid);
-  length->set_is_immutable(!FLAG_support_externalizable_strings);
+  length->set_is_immutable(true);
   length->set_recognized_kind(MethodRecognizer::kStringBaseLength);
 
   cursor = flow_graph->AppendTo(cursor, length, NULL, FlowGraph::kValue);