Change dependency walker's Node.dependencies to a static getter.

pkg/front_end/lib/src/dependency_walker.dart

Index: pkg/front_end/lib/src/dependency_walker.dart
diff --git a/pkg/front_end/lib/src/dependency_walker.dart b/pkg/front_end/lib/src/dependency_walker.dart
index 7a27acb7b7f5d372c770f0443cada115e92d500e..e00066ffc690ae3dc64c71008db33e79e4cc9fc5 100644
--- a/pkg/front_end/lib/src/dependency_walker.dart
+++ b/pkg/front_end/lib/src/dependency_walker.dart
@@ -26,11 +26,6 @@ abstract class Node<NodeType> {
   List<NodeType> _dependencies;
 
   /**
-   * Retrieve the dependencies of this node.
-   */
-  List<NodeType> get dependencies => _dependencies ??= computeDependencies();
-
-  /**
    * Indicates whether this node has been evaluated yet.
    */
   bool get isEvaluated;
@@ -39,6 +34,13 @@ abstract class Node<NodeType> {
    * Compute the dependencies of this node.
    */
   List<NodeType> computeDependencies();
+
+  /**
+   * Gets the dependencies of the given node, computing them if necessary.
+   */
+  static List<NodeType> getDependencies<NodeType>(Node<NodeType> node) {
+    return node._dependencies ??= node.computeDependencies();
+  }
 }
 
 /**
@@ -97,7 +99,7 @@ abstract class DependencyWalker<NodeType extends Node<NodeType>> {
       stack.add(node);
 
       // Consider the node's dependencies one at a time.
-      for (NodeType dependency in node.dependencies) {
+      for (NodeType dependency in Node.getDependencies(node)) {
         // If the dependency has already been evaluated, it can't be
         // part of this node's strongly connected component, so we can
         // skip it.