Account for @pragma("vm:entry-point") creating additional "root" libraries when partitioning the program into loading units.

If a library contained an entry point but was not reachable from the root library, it was not assigned to any loading unit and caused a null dereference in gen_snapshot. This is not possible with the standalone embedder, but is possible in Flutter because it passes multiple sources to frontend_server.  E.g., `--source dart_plugin_registrant.dart`.

TEST=gallery
Bug: https://github.com/flutter/gallery/issues/545
Change-Id: I9c67f0e39f7509094ee873610d80851a702a0cf2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249640
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

pkg/vm/lib/kernel_front_end.dart

@@ -509,7 +509,7 @@ Future runGlobalTransformations(
   // it does it will need the obfuscation prohibitions.
   obfuscationProhibitions.transformComponent(component, coreTypes, target);
 
-  deferred_loading.transformComponent(component);
+  deferred_loading.transformComponent(component, coreTypes, target);
 }
 
 /// Runs given [action] with [CompilerContext]. This is needed to

pkg/vm/lib/transformations/deferred_loading.dart

@@ -5,8 +5,12 @@
 library vm.transformations.deferred_loading;
 
 import 'package:kernel/ast.dart';
+import 'package:kernel/core_types.dart' show CoreTypes;
+import 'package:kernel/target/targets.dart' show Target;
+
 import '../dominators.dart';
 import '../metadata/loading_units.dart';
+import 'pragma.dart';
 
 class _LoadingUnitBuilder {
   late int id;
@@ -37,7 +41,42 @@ class _LibraryVertex extends Vertex<_LibraryVertex> {
   String toString() => "_LibraryVertex(${library.importUri})";
 }
 
-List<LoadingUnit> computeLoadingUnits(Component component) {
+class HasEntryPointVisitor extends RecursiveVisitor {
+  final PragmaAnnotationParser parser;
+  bool _hasEntryPoint = false;
+
+  HasEntryPointVisitor(this.parser);
+
+  visitAnnotations(List<Expression> annotations) {
+    for (var ann in annotations) {
+      ParsedPragma? pragma = parser.parsePragma(ann);
+      if (pragma is ParsedEntryPointPragma) {
+        _hasEntryPoint = true;
+        return;
+      }
+    }
+  }
+
+  @override
+  visitClass(Class klass) {
+    visitAnnotations(klass.annotations);
+    klass.visitChildren(this);
+  }
+
+  @override
+  defaultMember(Member node) {
+    visitAnnotations(node.annotations);
+  }
+
+  bool hasEntryPoint(Library lib) {
+    _hasEntryPoint = false;
+    visitLibrary(lib);
+    return _hasEntryPoint;
+  }
+}
+
+List<LoadingUnit> computeLoadingUnits(
+    Component component, HasEntryPointVisitor visitor) {
   // 1. Build the dominator tree for the library import graph.
   final map = <Library, _LibraryVertex>{};
   for (final lib in component.libraries) {
@@ -51,10 +90,15 @@ List<LoadingUnit> computeLoadingUnits(Component component) {
   }
   final root = map[component.mainMethod!.enclosingLibrary]!;
 
-  // Fake imports from root library to every core library so they end up in
-  // the same loading unit attributed to the user's root library.
+  // Fake imports from root library to every core library or library containing
+  // an entry point pragma so that they end up in the same loading unit
+  // attributed to the user's root library.
   for (final vertex in map.values) {
-    if (vertex.library.importUri.isScheme("dart")) {
+    if (vertex == root) {
+      continue;
+    }
+    if (vertex.library.importUri.isScheme("dart") ||
+        visitor.hasEntryPoint(vertex.library)) {
       root.successors.add(vertex);
       vertex.isLoadingRoot = false;
     }
@@ -127,8 +171,12 @@ List<LoadingUnit> computeLoadingUnits(Component component) {
   return loadingUnits.map((u) => u.asLoadingUnit()).toList();
 }
 
-Component transformComponent(Component component) {
-  final metadata = new LoadingUnitsMetadata(computeLoadingUnits(component));
+Component transformComponent(
+    Component component, CoreTypes coreTypes, Target target) {
+  final parser = ConstantPragmaAnnotationParser(coreTypes, target);
+  final visitor = HasEntryPointVisitor(parser);
+  final metadata =
+      new LoadingUnitsMetadata(computeLoadingUnits(component, visitor));
   final repo = new LoadingUnitsMetadataRepository();
   component.addMetadataRepository(repo);
   repo.mapping[component] = metadata;

pkg/vm/test/transformations/deferred_loading_test.dart

@@ -6,6 +6,7 @@ import 'dart:io';
 
 import 'package:kernel/target/targets.dart';
 import 'package:kernel/ast.dart';
+import 'package:kernel/core_types.dart';
 import 'package:kernel/kernel.dart';
 import 'package:test/test.dart';
 import 'package:vm/transformations/deferred_loading.dart'
@@ -24,7 +25,9 @@ runTestCase(Uri source) async {
   final reversed = component.libraries.reversed.toList();
   component.libraries.setAll(0, reversed);
 
-  component = transformComponent(component);
+  final coreTypes = CoreTypes(component);
+
+  component = transformComponent(component, coreTypes, target);
 
   // Remove core libraries so the expected output isn't enormous and broken by
   // core libraries changes.

runtime/vm/compiler/frontend/kernel_translation_helper.cc

@@ -1930,6 +1930,16 @@ void LoadingUnitsMetadataHelper::ReadMetadata(intptr_t node_offset) {
   object_store->set_loading_units(loading_units);
   ASSERT(object_store->loading_unit_uris() == Array::null());
   object_store->set_loading_unit_uris(loading_unit_uris);
+
+  const GrowableObjectArray& libraries =
+      GrowableObjectArray::Handle(zone, object_store->libraries());
+  for (intptr_t i = 0; i < libraries.Length(); i++) {
+    lib ^= libraries.At(i);
+    unit = lib.loading_unit();
+    if (unit.IsNull()) {
+      FATAL("%s is not attributed to any loading unit", lib.ToCString());
+    }
+  }
 }
 
 CallSiteAttributesMetadataHelper::CallSiteAttributesMetadataHelper(