null_safety rebuild script (#1369)

null_safety/null_unsafe_app/windows/runner/CMakeLists.txt

@@ -1,6 +1,11 @@
 cmake_minimum_required(VERSION 3.14)
 project(runner LANGUAGES CXX)
 
+# Define the application target. To change its name, change BINARY_NAME in the
+# top-level CMakeLists.txt, not the value here, or `flutter run` will no longer
+# work.
+#
+# Any new source files that you add to the application should be added here.
 add_executable(${BINARY_NAME} WIN32
   "flutter_window.cpp"
   "main.cpp"
@@ -10,8 +15,18 @@ add_executable(${BINARY_NAME} WIN32
   "Runner.rc"
   "runner.exe.manifest"
 )
+
+# Apply the standard set of build settings. This can be removed for applications
+# that need different build settings.
 apply_standard_settings(${BINARY_NAME})
+
+# Disable Windows macros that collide with C++ standard library functions.
 target_compile_definitions(${BINARY_NAME} PRIVATE "NOMINMAX")
+
+# Add dependency libraries and include directories. Add any application-specific
+# dependencies here.
 target_link_libraries(${BINARY_NAME} PRIVATE flutter flutter_wrapper_app)
 target_include_directories(${BINARY_NAME} PRIVATE "${CMAKE_SOURCE_DIR}")
+
+# Run the Flutter tool portions of the build. This must not be removed.
 add_dependencies(${BINARY_NAME} flutter_assemble)

null_safety/null_unsafe_app/windows/runner/utils.cpp

@@ -48,10 +48,10 @@ std::string Utf8FromUtf16(const wchar_t* utf16_string) {
   int target_length = ::WideCharToMultiByte(
       CP_UTF8, WC_ERR_INVALID_CHARS, utf16_string,
       -1, nullptr, 0, nullptr, nullptr);
-  if (target_length == 0) {
-    return std::string();
-  }
   std::string utf8_string;
+  if (target_length == 0 || target_length > utf8_string.max_size()) {
+    return utf8_string;
+  }
   utf8_string.resize(target_length);
   int converted_length = ::WideCharToMultiByte(
       CP_UTF8, WC_ERR_INVALID_CHARS, utf16_string,