Expose the dart kernel snapshot target and copied assets as a public dependency (#16266)

This allows for targets to depend on the assets generated by the test_fixtures target.

testing/testing.gni

@@ -162,7 +162,8 @@ template("dart_snapshot") {
     dart_kernel = dart_snapshot_kernel_path
   }
 
-  snapshot_deps = [ ":$dart_snapshot_kernel_target_name" ]
+  snapshot_deps = []
+  snapshot_public_deps = [ ":$dart_snapshot_kernel_target_name" ]
 
   if (is_aot_test) {
     dart_snapshot_aot_target_name = "dart_snapshot_aot_$target_name"
@@ -177,8 +178,8 @@ template("dart_snapshot") {
 
   group(target_name) {
     testonly = true
-
     deps = snapshot_deps
+    public_deps = snapshot_public_deps
   }
 }
 
@@ -218,6 +219,12 @@ template("copy_fixtures") {
 # unused invoker scope error. In such cases specify the fixtures using an empty
 # array.
 #
+# The targets which generate the outputs from these test fixtures (e.g. the
+# Dart kernel snapshot) are exposed as public dependencies of the test fixture
+# target. This is so that users can depend on the test fixture target directly
+# and be able to access the generated outputs without needing to know about the
+# internal dependency structure generated by this template.
+#
 # Arguments:
 #
 #     fixtures (optional): The list of test fixtures. An empty list may be
@@ -237,6 +244,7 @@ template("test_fixtures") {
     }
   }
   test_deps = [ ":$fixtures_location_target_name" ]
+  test_public_deps = []
 
   # If the fixtures are specified, copy them to the assets directory.
   if (defined(invoker.fixtures)) {
@@ -244,7 +252,7 @@ template("test_fixtures") {
     copy_fixtures(copy_fixtures_target_name) {
       fixtures = invoker.fixtures
     }
-    test_deps += [ ":$copy_fixtures_target_name" ]
+    test_public_deps += [ ":$copy_fixtures_target_name" ]
   }
 
   # If a Dart file is specified, snapshot it and place it in the generated
@@ -254,11 +262,12 @@ template("test_fixtures") {
     dart_snapshot(dart_snapshot_target_name) {
       dart_main = invoker.dart_main
     }
-    test_deps += [ ":$dart_snapshot_target_name" ]
+    test_public_deps += [ ":$dart_snapshot_target_name" ]
   }
 
   group(target_name) {
     testonly = true
     deps = test_deps
+    public_deps = test_public_deps
   }
 }