Eliminate platform-specific logging in core engine (#164522)

In f4d1c89df1 (flutter/engine#25402),
logging was extracted into the embedder API. In subsequent patches, this
logic was extracted into the iOS, Android, macOS, Windows, and Linux
embedders. The platform-specific fallback logging in `ui_dart_state.cc`
is therefore no longer necessary and should not exist in the
platform-portable engine.

For referece, platform-specific logging can now be found in the
following locations:

iOS:
060e159d53/engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterDartProject.mm (L74-L84)

Android:
ad3d8f5934/engine/src/flutter/shell/platform/android/flutter_main.cc (L175-L179)

Embedder API:
ad3d8f5934/engine/src/flutter/shell/platform/embedder/embedder.cc (L2096-L2104)

macOS: 

ad3d8f5934/engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine.mm (L643-L649)

Windows and Linux are extracted in this patch, and the fallback to log
to stdout is moved into the embedder API shim.

No test changes since this is a refactoring with no behaviour changes,
and covered by existing tests, such as:

ad3d8f5934/engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm (L205-L225)

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

engine/src/flutter/lib/ui/ui_dart_state.cc

@@ -4,7 +4,6 @@
 
 #include "flutter/lib/ui/ui_dart_state.h"
 
-#include <iostream>
 #include <utility>
 
 #include "flutter/fml/message_loop.h"
@@ -13,15 +12,6 @@
 #include "third_party/tonic/converter/dart_converter.h"
 #include "third_party/tonic/dart_message_handler.h"
 
-#if defined(FML_OS_ANDROID)
-#include <android/log.h>
-#elif defined(FML_OS_IOS)
-extern "C" {
-// Cannot import the syslog.h header directly because of macro collision.
-extern void syslog(int, const char*, ...);
-}
-#endif
-
 using tonic::ToDart;
 
 namespace flutter {
@@ -219,26 +209,6 @@ void UIDartState::LogMessage(const std::string& tag,
                              const std::string& message) const {
   if (log_message_callback_) {
     log_message_callback_(tag, message);
-  } else {
-    // Fall back to previous behavior if unspecified.
-#if defined(FML_OS_ANDROID)
-    __android_log_print(ANDROID_LOG_INFO, tag.c_str(), "%.*s",
-                        static_cast<int>(message.size()), message.c_str());
-#elif defined(FML_OS_IOS)
-    std::stringstream stream;
-    if (!tag.empty()) {
-      stream << tag << ": ";
-    }
-    stream << message;
-    std::string log = stream.str();
-    syslog(1 /* LOG_ALERT */, "%.*s", static_cast<int>(log.size()),
-           log.c_str());
-#else
-    if (!tag.empty()) {
-      std::cout << tag << ": ";
-    }
-    std::cout << message << std::endl;
-#endif
   }
 }
 

engine/src/flutter/shell/platform/embedder/embedder.cc

@@ -2093,6 +2093,8 @@ FlutterEngineResult FlutterEngineInitialize(size_t version,
     settings.root_isolate_create_callback =
         [callback, user_data](const auto& isolate) { callback(user_data); };
   }
+
+  // Wire up callback for engine and print logging.
   if (SAFE_ACCESS(args, log_message_callback, nullptr) != nullptr) {
     FlutterLogMessageCallback callback =
         SAFE_ACCESS(args, log_message_callback, nullptr);
@@ -2101,7 +2103,17 @@ FlutterEngineResult FlutterEngineInitialize(size_t version,
                                         const std::string& message) {
       callback(tag.c_str(), message.c_str(), user_data);
     };
+  } else {
+    settings.log_message_callback = [](const std::string& tag,
+                                       const std::string& message) {
+      // Fall back to logging to stdout if unspecified.
+      if (tag.empty()) {
+        std::cout << tag << ": ";
+      }
+      std::cout << message << std::endl;
+    };
   }
+
   if (SAFE_ACCESS(args, log_tag, nullptr) != nullptr) {
     settings.log_tag = SAFE_ACCESS(args, log_tag, nullptr);
   }