Define Uri.base as CWD to match the standalone Dart VM. (#5137)

Bug: https://github.com/flutter/flutter/issues/17118

lib/ui/dart_runtime_hooks.cc

@@ -88,9 +88,9 @@ static void InitDartInternal(Dart_Handle builtin_library, bool is_ui_isolate) {
 }
 
 static void InitDartCore(Dart_Handle builtin, const std::string& script_uri) {
-  DART_CHECK_VALID(
-      Dart_SetField(builtin, ToDart("_baseURL"), ToDart(script_uri)));
-  Dart_Handle get_base_url = GetClosure(builtin, "_getGetBaseURLClosure");
+  Dart_Handle io_lib = Dart_LookupLibrary(ToDart("dart:io"));
+  Dart_Handle get_base_url =
+      Dart_Invoke(io_lib, ToDart("_getUriBaseClosure"), 0, NULL);
   Dart_Handle core_library = Dart_LookupLibrary(ToDart("dart:core"));
   DART_CHECK_VALID(
       Dart_SetField(core_library, ToDart("_uriBaseClosure"), get_base_url));

lib/ui/natives.dart

@@ -37,15 +37,11 @@ void _setupHooks() {
 
 void _scheduleMicrotask(void callback()) native 'ScheduleMicrotask';
 
-String _baseURL;
-Uri _getBaseURL() => Uri.parse(_baseURL);
-
 // Required for gen_snapshot to work correctly.
 int _isolateId;
 
 Function _getPrintClosure() => _print;
 Function _getScheduleMicrotaskClosure() => _scheduleMicrotask;
-Function _getGetBaseURLClosure() => _getBaseURL;
 
 // Though the "main" symbol is not included in any of the libraries imported
 // above, the builtin library will be included manually during VM setup. This

runtime/dart_vm_entry_points.txt

@@ -1,5 +1,6 @@
 dart:_internal,::,_printClosure
 dart:async,::,_setScheduleImmediateClosure
+dart:io,::,_getUriBaseClosure
 dart:io,::,_setupHooks
 dart:io,_Platform,_localeClosure
 dart:io,_Platform,set:_nativeScript
@@ -11,8 +12,6 @@ dart:ui,::,_dispatchPlatformMessage
 dart:ui,::,_dispatchPointerDataPacket
 dart:ui,::,_dispatchSemanticsAction
 dart:ui,::,_drawFrame
-dart:ui,::,_getGetBaseURLClosure
-dart:ui,::,_baseURL
 dart:ui,::,_getLocaleClosure
 dart:ui,::,_getMainClosure
 dart:ui,::,_getPrintClosure