Here is a list of all namespace members with links to the namespace documentation for each member:

UintPoint32 : impeller
UIThreadPolicy : flutter
UnblockSIGPROF() : flutter
UndeadChar() : flutter
UnhandledExceptionCallback : flutter
UnhandledExceptionReporter : tonic::DartError
UnimplementedNativeViewAccessible : gfx
UnionRects() : gfx
UniqueAllocatorVMA : impeller
UniqueAOTData : flutter::testing
UniqueAotDataPtr : flutter
UniqueBufferVMA : impeller
UniqueDir : fml
UniqueEGLImage : impeller
UniqueEGLImageKHR : impeller
UniqueEngine : flutter::testing
UniqueFD : fml
UniqueFDIONS : flutter_runner
UniqueFDIONSCreate() : flutter_runner
UniqueGLTexture : impeller
UniqueIdMap : ui
UniqueImageVMA : impeller
UniqueLoadedELF : flutter::testing
UniquePoolVMA : impeller
Unit() : flutter
UnitlessTime : flutter::testing
UnlinkDirectory() : fml
UnlinkFile() : fml
UnpackPointerPacket() : flutter::testing
UnreachableScopeWithMacroWithoutReturnDoesNotMakeCompilerMad() : base::testing, fml::testing
UnreachableScopeWithoutReturnDoesNotMakeCompilerMad() : base::testing, fml::testing
UnregisterTexture() : flutter
UnserializeKeyEventDeviceType() : flutter::testing
UnserializeKeyEventType() : flutter::testing
UpdateAtlasBitmap() : impeller
UpdateDisplayMetrics() : flutter
UpdateJavaAssetManager() : flutter
UploadRasterImage() : flutter
Utf16StringConverter : fml
UTF16ToUTF8() : base
Utf16ToUtf8() : fml
Utf16ToWideString() : fml
Utf8FromPath() : impeller::compiler
UTF8ToUTF16() : base
Utf8ToUtf16() : fml
Utf8ToWideString() : fml