[vm] Don't write LibraryPrefix::importer_ in AOT snapshots.

This field won't be used at runtime and is not accounted for during tree-shaking.

Bug: dart-lang#41974
Bug: dart-lang#43245
Change-Id: Ie5a669608e896aa8cee74a3a4249ba99ec6f980a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/161220
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/raw_object.h

@@ -2143,15 +2143,16 @@ class LibraryPrefixLayout : public InstanceLayout {
 
   VISIT_FROM(ObjectPtr, name_)
   StringPtr name_;       // Library prefix name.
-  LibraryPtr importer_;  // Library which declares this prefix.
   ArrayPtr imports_;     // Libraries imported with this prefix.
-  VISIT_TO(ObjectPtr, imports_)
+  LibraryPtr importer_;  // Library which declares this prefix.
+  VISIT_TO(ObjectPtr, importer_)
   ObjectPtr* to_snapshot(Snapshot::Kind kind) {
     switch (kind) {
-      case Snapshot::kFull:
-      case Snapshot::kFullJIT:
       case Snapshot::kFullAOT:
         return reinterpret_cast<ObjectPtr*>(&imports_);
+      case Snapshot::kFull:
+      case Snapshot::kFullJIT:
+        return reinterpret_cast<ObjectPtr*>(&importer_);
       case Snapshot::kMessage:
       case Snapshot::kNone:
       case Snapshot::kInvalid:

tests/language/deferred/prefix_importer_tree_shaken_deferred.dart

@@ -0,0 +1,8 @@
+// Copyright (c) 2020, 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.
+
+@pragma("vm:never-inline")
+bar() {
+  return "bar";
+}

tests/language/deferred/prefix_importer_tree_shaken_immediate.dart

@@ -0,0 +1,11 @@
+// Copyright (c) 2020, 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.
+
+import "prefix_importer_tree_shaken_deferred.dart" deferred as d;
+
+@pragma("vm:prefer-inline")
+load() => d.loadLibrary();
+
+@pragma("vm:prefer-inline")
+foo() => d.bar();

tests/language/deferred/prefix_importer_tree_shaken_test.dart

@@ -0,0 +1,13 @@
+// Copyright (c) 2020, 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.
+
+/// VMOptions=--dwarf_stack_traces=true
+/// VMOptions=--dwarf_stack_traces=false
+
+import "prefix_importer_tree_shaken_immediate.dart" as i;
+
+main() async {
+  await i.load();
+  print(await i.foo());
+}

tests/language_2/deferred/prefix_importer_tree_shaken_deferred.dart

@@ -0,0 +1,8 @@
+// Copyright (c) 2020, 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.
+
+@pragma("vm:never-inline")
+bar() {
+  return "bar";
+}

tests/language_2/deferred/prefix_importer_tree_shaken_immediate.dart

@@ -0,0 +1,11 @@
+// Copyright (c) 2020, 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.
+
+import "prefix_importer_tree_shaken_deferred.dart" deferred as d;
+
+@pragma("vm:prefer-inline")
+load() => d.loadLibrary();
+
+@pragma("vm:prefer-inline")
+foo() => d.bar();

tests/language_2/deferred/prefix_importer_tree_shaken_test.dart

@@ -0,0 +1,13 @@
+// Copyright (c) 2020, 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.
+
+/// VMOptions=--dwarf_stack_traces=true
+/// VMOptions=--dwarf_stack_traces=false
+
+import "prefix_importer_tree_shaken_immediate.dart" as i;
+
+main() async {
+  await i.load();
+  print(await i.foo());
+}