[dart2js] Cache deferred JS AST bodies during printing and source map generation.

There was a bug in source map generation introduced by this deferring. A map from JS Nodes to source map info is being shared between these two operations and in order to look up nodes in the map, the Node objects must be the same.

This gives up some but not all of the gains from deferring these function bodies. I'm working on a more long term fix that combines these two passes of the AST.

Bugs: https://github.com/dart-lang/sdk/issues/53466
Change-Id: If25d1b24dd32456155e8802725d480454c71d2b6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/325600
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>

pkg/compiler/lib/src/js/js.dart

@@ -55,12 +55,48 @@ CodeBuffer createCodeBuffer(Node node, CompilerOptions compilerOptions,
 
   Dart2JSJavaScriptPrintingContext context = Dart2JSJavaScriptPrintingContext(
       monitor, outBuffer, sourceInformationProcessor, annotationMonitor);
+
+  /// We defer deserialization of function bodies but maintain maps using
+  /// nodes as keys for source map generation. In order to ensure the map's
+  /// references are the same between printing and source map generation we
+  /// cache the contents of the deferred blocks during these two operations.
+  final deferredBlockCollector = _CollectDeferredBlocksAndSetCaches();
+  deferredBlockCollector.setCache(node);
   Printer printer = Printer(options, context);
   printer.visit(node);
   sourceInformationProcessor.process(node, outBuffer);
+  deferredBlockCollector.clearCache();
   return outBuffer;
 }
 
+class _CollectDeferredBlocksAndSetCaches extends BaseVisitorVoid {
+  final List<DeferredBlock> _blocks = [];
+
+  _CollectDeferredBlocksAndSetCaches();
+
+  void setCache(Node node) {
+    node.accept(this);
+  }
+
+  void clearCache() {
+    _blocks.forEach((block) => block._clearCache());
+    _blocks.clear();
+  }
+
+  @override
+  void visitBlock(Block node) {
+    if (node is DeferredBlock) {
+      if (!node._isCached) {
+        _blocks.add(node);
+        node._setCache();
+        super.visitBlock(node);
+      }
+    } else {
+      super.visitBlock(node);
+    }
+  }
+}
+
 class JavaScriptAnnotationMonitor {
   const JavaScriptAnnotationMonitor();
 
@@ -268,8 +304,19 @@ class DeferredExpressionData {
 /// Each time [statements] is invoked, the enclosed [Statement] list will be
 /// deserialized so care should be taken to limit this.
 class DeferredBlock extends Statement implements Block {
+  bool hit = false;
+  List<Statement> getLoaded() {
+    return _statements.loaded();
+  }
+
+  List<Statement>? _cached;
+
+  void _setCache() => _cached = getLoaded();
+  void _clearCache() => _cached = null;
+  bool get _isCached => _cached != null;
+
   @override
-  List<Statement> get statements => _statements.loaded();
+  List<Statement> get statements => _cached ?? getLoaded();
 
   final Deferrable<List<Statement>> _statements;