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

https://github.com/domokit/mojo/issues/98

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1088263004

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();

tests/dart/mirror-system-expected.txt

+PASS

tests/dart/mirror-system.sky

+<script>
+import "dart:mirrors";
+import "dart:sky.internals" as internals;
+
+// Regression test for the embedder not setting up the root library.
+// Used to crash.
+
+void main() {
+  if (currentMirrorSystem().isolate.rootLibrary == null) {
+    throw "Missing root library";
+  }
+  internals.notifyTestComplete("PASS");
+}
+</script>>