Add new code for resolving deps graphs in a phased build.

Author: davidmorgan

build/lib/src/deps_graph/deps_cycle.dart

@@ -0,0 +1,17 @@
+// Copyright (c) 2025, 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 'package:built_collection/built_collection.dart';
+import 'package:built_value/built_value.dart';
+
+import '../asset/id.dart';
+
+part 'deps_cycle.g.dart';
+
+abstract class DepsCycle implements Built<DepsCycle, DepsCycleBuilder> {
+  BuiltSet<AssetId> get nodes;
+
+  factory DepsCycle([void Function(DepsCycleBuilder) updates]) = _$DepsCycle;
+  DepsCycle._();
+}

build/lib/src/deps_graph/deps_cycle.g.dart

@@ -0,0 +1,35 @@
+// Copyright (c) 2025, 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 'package:built_collection/built_collection.dart';
+import 'package:built_value/built_value.dart';
+
+import '../asset/id.dart';
+import 'deps_cycle.dart';
+
+part 'deps_graph.g.dart';
+
+abstract class DepsGraph implements Built<DepsGraph, DepsGraphBuilder> {
+  DepsCycle get root;
+  BuiltList<DepsGraph> get children;
+
+  factory DepsGraph([void Function(DepsGraphBuilder) updates]) = _$DepsGraph;
+  DepsGraph._();
+
+  // TODO(davidmorgan): the graph should usually stay as a graph rather than
+  // being expanded into an explicit set of nodes. So, remove uses of this.
+  // If in the end it's still needed, investigate if it needs to be optimized.
+  Iterable<AssetId> get transitiveDeps sync* {
+    final seenCycles = Set<DepsCycle>.identity()..add(root);
+    final graphs = [this];
+
+    while (graphs.isNotEmpty) {
+      final graph = graphs.removeLast();
+      if (seenCycles.add(graph.root)) {
+        yield* graph.root.nodes;
+      }
+      graphs.addAll(graph.children);
+    }
+  }
+}