Add inference test functionality based on code point id.

tests/compiler/dart2js/inference/enumerator.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.
+
+import 'package:compiler/src/resolution/access_semantics.dart';
+import 'package:compiler/src/resolution/send_structure.dart';
+import 'package:compiler/src/resolution/tree_elements.dart';
+import 'package:compiler/src/tree/nodes.dart' as ast;
+
+/// Id for a code point with type inference information.
+// TODO(johnniwinther): Create an [Id]-based equivalence with the kernel IR.
+class Id {
+  final int value;
+
+  const Id(this.value);
+
+  int get hashCode => value.hashCode;
+
+  bool operator ==(other) {
+    if (identical(this, other)) return true;
+    if (other is! Id) return false;
+    return value == other.value;
+  }
+
+  String toString() => value.toString();
+}
+
+abstract class AstEnumeratorMixin {
+  TreeElements get elements;
+
+  Id computeAccessId(ast.Send node, AccessSemantics access) {
+    switch (access.kind) {
+      case AccessKind.DYNAMIC_PROPERTY:
+        return new Id(node.selector.getBeginToken().charOffset);
+      default:
+        return new Id(node.getBeginToken().charOffset);
+    }
+  }
+
+  Id computeId(ast.Send node) {
+    var sendStructure = elements.getSendStructure(node);
+    if (sendStructure == null) return null;
+    switch (sendStructure.kind) {
+      case SendStructureKind.GET:
+      case SendStructureKind.INVOKE:
+      case SendStructureKind.INCOMPATIBLE_INVOKE:

[Siggi Cherem (dart-lang), 2017/01/24 21:05:59]

any need to include SET too?

[Johnni Winther, 2017/01/25 13:28:13]

Not sure. I'm waiting for dartk to produce file offsets in Kernel IR. Currently
they use the offset of the left-hand side.

+      case SendStructureKind.INCOMPATIBLE_INVOKE:

[Siggi Cherem (dart-lang), 2017/01/24 21:05:59]

remove dup?

[Johnni Winther, 2017/01/25 13:28:13]

Ok, then ;-)

+        return computeAccessId(node, sendStructure.semantics);
+      default:
+        return new Id(node.getBeginToken().charOffset);
+    }
+  }
+}