Ensure isolates have a root library so createMirrorSystem() and spawnUri() are happy.

sky/engine/core/script/dart_controller.cc

Index: sky/engine/core/script/dart_controller.cc
diff --git a/sky/engine/core/script/dart_controller.cc b/sky/engine/core/script/dart_controller.cc
index 2a05e58855233c32072551cf6226ece0e1de2935..dc7ce4c78a63d901d5517e4a7b55e984c7671a8d 100644
--- a/sky/engine/core/script/dart_controller.cc
+++ b/sky/engine/core/script/dart_controller.cc
@@ -204,6 +204,10 @@ static Dart_Isolate IsolateCreateCallback(const char* script_uri,
     DartApiScope apiScope;
     Builtin::SetNativeResolver(Builtin::kBuiltinLibrary);
     Builtin::SetNativeResolver(Builtin::kMojoInternalLibrary);
+
+    // Ensure the isolate has a root library.
+    Dart_LoadScript(Dart_NewStringFromCString("dart:empty"),
+                    Dart_NewStringFromCString(""), 0, 0);
   }
 
   Dart_ExitIsolate();
@@ -273,6 +277,10 @@ void DartController::CreateIsolateFor(Document* document) {
   {
     DartApiScope apiScope;
 
+    // Ensure the isolate has a root library.
+    Dart_LoadScript(Dart_NewStringFromCString("dart:empty"),
+                    Dart_NewStringFromCString(""), 0, 0);
+
     Builtin::SetNativeResolver(Builtin::kBuiltinLibrary);
     Builtin::SetNativeResolver(Builtin::kMojoInternalLibrary);
     BuiltinNatives::Init();