Move AbstractNode to lib/foundation. (#12789)

(in preparation for moving the semantics stuff out of rendering)

packages/flutter/lib/foundation.dart

@@ -40,6 +40,7 @@ export 'src/foundation/collections.dart';
 export 'src/foundation/debug.dart';
 export 'src/foundation/diagnostics.dart';
 export 'src/foundation/licenses.dart';
+export 'src/foundation/node.dart';
 export 'src/foundation/observer_list.dart';
 export 'src/foundation/platform.dart';
 export 'src/foundation/print.dart';

packages/flutter/lib/rendering.dart

@@ -42,7 +42,6 @@ export 'src/rendering/flow.dart';
 export 'src/rendering/image.dart';
 export 'src/rendering/layer.dart';
 export 'src/rendering/list_body.dart';
-export 'src/rendering/node.dart';
 export 'src/rendering/object.dart';
 export 'src/rendering/paragraph.dart';
 export 'src/rendering/performance_overlay.dart';

packages/flutter/lib/src/rendering/node.dart → packages/flutter/lib/src/foundation/node.dart

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter/foundation.dart';
+import 'package:meta/meta.dart';
 
 /// An abstract node in a tree.
 ///

packages/flutter/lib/src/rendering/layer.dart

@@ -11,7 +11,6 @@ import 'package:flutter/painting.dart';
 import 'package:vector_math/vector_math_64.dart';
 
 import 'debug.dart';
-import 'node.dart';
 
 /// A composited layer.
 ///

packages/flutter/lib/src/rendering/object.dart

@@ -14,7 +14,6 @@ import 'package:vector_math/vector_math_64.dart';
 import 'binding.dart';
 import 'debug.dart';
 import 'layer.dart';
-import 'node.dart';
 import 'semantics.dart';
 
 export 'package:flutter/foundation.dart' show FlutterError, InformationCollector, DiagnosticsNode, DiagnosticsProperty, StringProperty, DoubleProperty, EnumProperty, FlagProperty, IntProperty, DiagnosticPropertiesBuilder;

packages/flutter/lib/src/rendering/semantics.dart

@@ -12,7 +12,6 @@ import 'package:flutter/services.dart';
 import 'package:vector_math/vector_math_64.dart';
 
 import 'debug.dart';
-import 'node.dart';
 import 'semantics_event.dart';
 
 export 'dart:ui' show SemanticsAction;

packages/flutter/test/rendering/non_render_object_root_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/material.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:test/test.dart';