Start implementing IncrementalClassHierarchy.

pkg/kernel/lib/src/incremental_class_hierarchy.dart

+// Copyright (c) 2017, 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.
+library kernel.incremental_class_hierarchy;
+
+import 'dart:math';
+
+import 'package:kernel/ast.dart';
+import 'package:kernel/class_hierarchy.dart';
+
+/// Lazy and incremental implementation of [ClassHierarchy].
+class IncrementalClassHierarchy implements ClassHierarchy {
+  /// The mapping from [Class]es to the corresponding [_ClassInfo]s.
+  /// It is filled lazily as the client requests information about classes.

[ahe, 2017/06/02 11:34:48]

What are your plans for the incremental part of this?

[scheglov, 2017/06/02 15:36:07]

I think we can use single instance of IncrementalClassHierarchy to sequentially
analyze all strong connected components of libraries. So, every new class we
will request information for is "increment" to already known information about
its superclasses defined in previous components.

For mixin processing, we might want to clear information about a class when we
make changes to its structure, e.g. copy methods into it.

[ahe, 2017/06/02 15:43:46]

Thank you for explaining.

+  final Map<Class, _ClassInfo> _info = {};
+
+  @override
+  Iterable<Class> get classes {
+    // TODO(scheglov): implement classes
+    throw new UnimplementedError();
+  }
+
+  @override
+  void forEachOverridePair(Class class_,
+      callback(Member declaredMember, Member interfaceMember, bool isSetter)) {
+    // TODO(scheglov): implement forEachOverridePair
+    throw new UnimplementedError();
+  }
+
+  @override
+  Supertype getClassAsInstanceOf(Class class_, Class superclass) {
+    // TODO(scheglov): implement getClassAsInstanceOf
+    throw new UnimplementedError();
+  }
+
+  @override
+  int getClassDepth(Class node) {
+    var info = _getInfo(node);
+    if (info.depth < 0) {
+      int superDepth = -1;
+      if (node.supertype != null) {
+        superDepth = max(superDepth, getClassDepth(node.supertype.classNode));

[ahe, 2017/06/02 11:34:48]

Any concerns about recursion here?

[Paul Berry, 2017/06/02 12:25:48]

FWIW the topological sort in ClosedWorldClassHierarchy has the same potential
issue, and AFAIK it has not caused any problems yet.

[ahe, 2017/06/02 12:29:03]

This situation might be slightly different. However, if you have called
SourceLoader.checkSemantics first, you should be fine.

+      }
+      if (node.mixedInType != null) {
+        superDepth = max(superDepth, getClassDepth(node.mixedInType.classNode));
+      }
+      for (var supertype in node.implementedTypes) {
+        superDepth = max(superDepth, getClassDepth(supertype.classNode));
+      }
+      info.depth = superDepth + 1;
+    }
+
+    return info.depth;
+  }
+
+  @override
+  InterfaceType getClassicLeastUpperBound(
+      InterfaceType type1, InterfaceType type2) {
+    // TODO(scheglov): implement getClassicLeastUpperBound
+    throw new UnimplementedError();
+  }
+
+  @override
+  int getClassIndex(Class class_) {
+    // TODO(scheglov): implement getClassIndex
+    throw new UnimplementedError();
+  }
+
+  @override
+  Member getDispatchTarget(Class class_, Name name, {bool setter: false}) {
+    // TODO(scheglov): implement getDispatchTarget
+    throw new UnimplementedError();
+  }
+
+  @override
+  Member getInterfaceMember(Class class_, Name name, {bool setter: false}) {
+    // TODO(scheglov): implement getInterfaceMember
+    throw new UnimplementedError();
+  }
+
+  @override
+  List<Class> getRankedSuperclasses(Class class_) {
+    // TODO(scheglov): implement getRankedSuperclasses
+    throw new UnimplementedError();
+  }
+
+  @override
+  InterfaceType getTypeAsInstanceOf(InterfaceType type, Class superclass) {
+    // TODO(scheglov): implement getTypeAsInstanceOf
+    throw new UnimplementedError();
+  }
+
+  @override
+  bool hasProperSubtypes(Class class_) {
+    // TODO(scheglov): implement hasProperSubtypes
+    throw new UnimplementedError();
+  }
+
+  /// Return the [_ClassInfo] for the [node].
+  _ClassInfo _getInfo(Class node) {
+    return _info.putIfAbsent(node, () => new _ClassInfo(node));
+  }
+}
+
+/// Information about a [Class].
+class _ClassInfo {
+  final Class node;
+
+  /// The number of steps in the longest inheritance path from the class
+  /// to [Object], or -1 if the depth has not been computed yet.
+  int depth = -1;
+
+  _ClassInfo(this.node);
+}