Add supporting macro for writing tests

ci/licenses_golden/licenses_flutter

@@ -1099,6 +1099,7 @@ FILE: ../../../flutter/shell/platform/embedder/fixtures/verifyb143464703.png
 FILE: ../../../flutter/shell/platform/embedder/fixtures/verifyb143464703_soft_noxform.png
 FILE: ../../../flutter/shell/platform/embedder/platform_view_embedder.cc
 FILE: ../../../flutter/shell/platform/embedder/platform_view_embedder.h
+FILE: ../../../flutter/shell/platform/embedder/test_utils/proc_table_replacement.h
 FILE: ../../../flutter/shell/platform/embedder/vsync_waiter_embedder.cc
 FILE: ../../../flutter/shell/platform/embedder/vsync_waiter_embedder.h
 FILE: ../../../flutter/shell/platform/fuchsia/dart-pkg/fuchsia/lib/fuchsia.dart

shell/platform/embedder/BUILD.gn

@@ -110,6 +110,14 @@ source_set("embedder_headers") {
   public_configs = [ "//flutter:config" ]
 }
 
+source_set("embedder_test_utils") {
+  public = [ "test_utils/proc_table_replacement.h" ]
+
+  deps = [ ":embedder_headers" ]
+
+  public_configs = [ "//flutter:config" ]
+}
+
 # For using the embedder API as internal implementation detail of an
 # embedding.
 config("embedder_internal_library_config") {

shell/platform/embedder/test_utils/proc_table_replacement.h

@@ -0,0 +1,23 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "flutter/shell/platform/embedder/embedder.h"
+
+// Wraps capturing lambas with non-capturing version that can be assigned to
+// FlutterEngineProcTable entries (by using statics) to facilitate mocking in
+// tests of code built on top of the embedder API.
+//
+// This should *ONLY* be used in unit tests as it is leaky by design.
+//
+// |proc| should be the name of an entry in FlutterEngineProcTable, such as
+// "initialize". |mock_impl| should be a lamba that replaces its implementation,
+// taking the same arguments and returning the same type.
+#define MOCK_ENGINE_PROC(proc, mock_impl)                                      \
+  ([&]() {                                                                     \
+    static std::function<                                                      \
+        std::remove_pointer_t<decltype(FlutterEngineProcTable::proc)>>         \
+        closure = mock_impl;                                                   \
+    static auto non_capturing = [](auto... args) { return closure(args...); }; \
+    return non_capturing;                                                      \
+  })()