[vm] Handle WSRs more generally in v8 snapshot profile writing.

Instead of special-casing the current fields that may have
WeakSerializationReferences, handle WSRs appearing as elements
or properties of objects more generally. This removes existing
special casing and avoids the need for it in case of new
future uses of WSRs.

For artificial nodes being added for dropped
WeakSerializationReference targets, add them as kArtificial nodes
(not kSnapshot) that has the original offset (element) or
name (property). The replacement is added as a kSnapshot node
that has a negative offset with the same magnitude as the
artificial node (element) or ":real_<property name>" (property).
This simplifies the work done in pkg/vm_snapshot_analysis to
use the artificial nodes instead of replacement ones for
reassembling hierarchies and the like.

This CL also cleans up the old SerializerWritingObjectScope
class, both moving it to Serializer::WritingObjectScope and
allowing nesting of WritingObjectScopes with the correct
semantics.

Thanks to this, not only can we recur when under a WritingObjectScope
instead of lifting recursion outside of those scopes, but we can
also create artificial nodes for non-empty per-code object pools and
static call target tables instead of attributing their contents
as supposed elements of the Code object being written.

TEST=pkg/vm_snapshot_analysis/test/instruction_sizes_test

Cq-Include-Trybots: luci.dart.try:pkg-linux-release-try,pkg-mac-release-try,pkg-win-release-try
Change-Id: Ib945c5afcd89b1458b8be3559b6eae24048aba2f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/194243
Commit-Queue: Tess Strickland <sstrickl@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>

pkg/vm_snapshot_analysis/lib/v8_profile.dart

@@ -88,6 +88,25 @@ class Snapshot {
         m['strings'],
         edgesStartIndexForNode);
   }
+
+  @override
+  String toString() {
+    final buffer = StringBuffer();
+    buffer
+      ..write("Node count: ")
+      ..writeln(nodeCount)
+      ..write("Edge count: ")
+      ..writeln(edgeCount);
+    buffer.write("Nodes:");
+    for (final node in nodes) {
+      buffer
+        ..writeln()
+        ..write(node.index)
+        ..write(': ')
+        ..writeln(node);
+    }
+    return buffer.toString();
+  }
 }
 
 /// Meta-information about the serialized snapshot.
@@ -228,12 +247,6 @@ class Node {
     }.toString();
   }
 
-  /// Returns the target of an outgoing edge with the given name (if any),
-  /// but first checks for a corresponding artificial edge indicating a dropped
-  /// object.
-  Node possiblyDroppedTarget(String edgeName) =>
-      this[':$edgeName'] ?? this[edgeName];
-
   /// Returns the target of an outgoing edge with the given name (if any).
   Node operator [](String edgeName) => this
       .edges
@@ -404,7 +417,7 @@ class _ProgramInfoBuilder {
   ProgramInfoNode createInfoNodeFor(Node node) {
     switch (node.type) {
       case 'Code':
-        var owner = node.possiblyDroppedTarget('owner_');
+        var owner = node['owner_'];
         if (owner.type != 'Type') {
           final ownerNode =
               owner.type == 'Null' ? program.stubs : getInfoNodeFor(owner);
@@ -428,7 +441,7 @@ class _ProgramInfoBuilder {
           // Artificial nodes may not have a data_ field.
           var data = node['data_'];
           if (data?.type == 'ClosureData') {
-            owner = data.possiblyDroppedTarget('parent_function_');
+            owner = data['parent_function_'];
           }
           return makeInfoNode(node.index,
               name: node.name,

pkg/vm_snapshot_analysis/test/utils.dart

@@ -60,17 +60,21 @@ void main(List<String> args) => input.main(args);
       if (flag != null) '$flag=${snapshot.sizesJson}',
     ];
 
-    // Compile input.dart to native and output instruction sizes.
-    final result = await Process.run(dart2native, [
+    final args = [
       '-o',
       snapshot.outputBinary,
       '--packages=$packages',
       '--extra-gen-snapshot-options=${extraGenSnapshotOptions.join(',')}',
       mainDart,
-    ]);
+    ];
+
+    // Compile input.dart to native and output instruction sizes.
+    final result = await Process.run(dart2native, args);
 
     expect(result.exitCode, equals(0), reason: '''
-Compilation completed successfully.
+Compilation completed with exit code ${result.exitCode}.
+
+Command line: $dart2native ${args.join(' ')}
 
 stdout: ${result.stdout}
 stderr: ${result.stderr}
@@ -86,13 +90,18 @@ stderr: ${result.stderr}
   });
 }
 
+const keepTempKey = 'KEEP_TEMPORARY_DIRECTORIES';
+
 Future withTempDir(Future Function(String dir) f) async {
   final tempDir =
       Directory.systemTemp.createTempSync('instruction-sizes-test-');
   try {
     await f(tempDir.path);
   } finally {
-    tempDir.deleteSync(recursive: true);
+    if (!Platform.environment.containsKey(keepTempKey) ||
+        Platform.environment[keepTempKey].isEmpty) {
+      tempDir.deleteSync(recursive: true);
+    }
   }
 }