Change the approach of front_end top level inference to match the spec.

pkg/front_end/lib/src/fasta/type_inference/dependency_collector.dart

Index: pkg/front_end/lib/src/fasta/type_inference/dependency_collector.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/dependency_collector.dart b/pkg/front_end/lib/src/fasta/type_inference/dependency_collector.dart
new file mode 100644
index 0000000000000000000000000000000000000000..5e331a8b5b16d8a0cf2669e9459125ead3391532
--- /dev/null
+++ b/pkg/front_end/lib/src/fasta/type_inference/dependency_collector.dart
@@ -0,0 +1,37 @@
+// 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.md file.
+
+import 'package:front_end/src/fasta/type_inference/type_inference_engine.dart';
+import 'package:kernel/ast.dart' show Expression;
+
+/// Keeps track of state while determining whether an expression has an
+/// immediately evident type, and if so what its dependencies are.
+///
+/// This class describes the interface for use by clients of type inference.
+/// The implementation is in [DependencyCollectorImpl].
+abstract class DependencyCollector {
+  /// Collects any dependencies of [expression], and reports errors if the
+  /// expression does not have an immediately evident type.
+  void collectDependencies(Expression expression);
+}
+
+/// Generic implementation of [DependencyCollectorImpl].
+///
+/// This class contains all of the implementation of [DependencyCollector] which
+/// can be expressed without the need to reference private data structures in
+/// the shadow hierarchy.
+abstract class DependencyCollectorImpl extends DependencyCollector {
+  List<FieldNode> dependencies = [];
+
+  bool isImmediatelyEvident = true;
+
+  void recordDependency(FieldNode fieldNode) {
+    dependencies.add(fieldNode);
+  }
+
+  void recordNotImmediatelyEvident(int fileOffset) {
+    isImmediatelyEvident = false;
+    // TODO(paulberry): report an error.
+  }
+}