HitTestResult.path should be unmodifiable (#4482)

Fixes #4428

packages/flutter/lib/src/gestures/hit_test.dart

@@ -47,14 +47,15 @@ class HitTestResult {
   /// If the [path] argument is null, the [path] field will be initialized with
   /// and empty list.
   HitTestResult({ List<HitTestEntry> path })
-    : path = path ?? <HitTestEntry>[];
+    : _path = path ?? <HitTestEntry>[];
 
-  /// The list of [HitTestEntry] objects recorded during the hit test.
+  /// An unmodifiable list of [HitTestEntry] objects recorded during the hit test.
   ///
   /// The first entry in the path is the most specific, typically the one at
   /// the leaf of tree being hit tested. Event propagation starts with the most
   /// specific (i.e., first) entry and proceeds in order through the path.
-  final List<HitTestEntry> path;
+  List<HitTestEntry> get path => new List<HitTestEntry>.unmodifiable(_path);
+  final List<HitTestEntry> _path;
 
   /// Add a [HitTestEntry] to the path.
   ///
@@ -62,9 +63,9 @@ class HitTestResult {
   /// be added in order from most specific to least specific, typically during an
   /// upward walk of the tree being hit tested.
   void add(HitTestEntry entry) {
-    path.add(entry);
+    _path.add(entry);
   }
 
   @override
-  String toString() => 'HitTestResult(${path.isEmpty ? "<empty path>" : path.join(", ")})';
+  String toString() => 'HitTestResult(${_path.isEmpty ? "<empty path>" : _path.join(", ")})';
 }