Fix optimizer bug with external strings.

runtime/vm/dart_api_impl_test.cc

Index: runtime/vm/dart_api_impl_test.cc
diff --git a/runtime/vm/dart_api_impl_test.cc b/runtime/vm/dart_api_impl_test.cc
index f6e8bb716f4663e322c2f776ed252ad5a34ffc84..fcc4e79cd8cc88cc39f412f13ed1490c827bd007 100644
--- a/runtime/vm/dart_api_impl_test.cc
+++ b/runtime/vm/dart_api_impl_test.cc
@@ -8957,6 +8957,41 @@ TEST_CASE(ExternalStringPolymorphicDeoptimize) {
 }
 
 
+TEST_CASE(ExternalStringLoadElimination) {
+  const bool saved_flag = FLAG_support_externalizable_strings;
+  FLAG_support_externalizable_strings = true;
+
+  const char* kScriptChars =
+      "class A {\n"
+      "  static change_str(String s) native 'A_change_str';\n"
+      "}\n"
+      "double_char0(str) {\n"
+      "  return str.codeUnitAt(0) + str.codeUnitAt(0);\n"
+      "}\n"
+      "main() {\n"
+      "  var externalA = 'AA' + 'AA';\n"
+      "  A.change_str(externalA);\n"
+      "  for (var i = 0; i < 10000; i++) double_char0(externalA);\n"
+      "  var result = double_char0(externalA);\n"
+      "  return result == 130;\n"
+      "}\n";
+  Dart_Handle lib =
+      TestCase::LoadTestScript(kScriptChars,
+                               &ExternalStringDeoptimize_native_lookup);
+  Dart_Handle result = Dart_Invoke(lib,
+                                   NewString("main"),
+                                   0,
+                                   NULL);
+  EXPECT_VALID(result);
+  bool value = false;
+  result = Dart_BooleanValue(result, &value);
+  EXPECT_VALID(result);
+  EXPECT(value);
+
+  FLAG_support_externalizable_strings = saved_flag;
+}
+
+
 TEST_CASE(ExternalStringGuardFieldDeoptimize) {
   const bool saved_flag = FLAG_support_externalizable_strings;
   FLAG_support_externalizable_strings = true;