Throw an exception when attempting to use Isolate.spawnUri under precompilation.

runtime/lib/isolate.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "include/dart_native_api.h"
 #include "platform/assert.h"
 #include "vm/bootstrap_natives.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart.h"
 #include "vm/dart_api_impl.h"
@@ skipping 299 matching lines @@
   GET_NATIVE_ARGUMENT(SendPort, onError, arguments->NativeArgAt(6));
 
   GET_NATIVE_ARGUMENT(Bool, fatalErrors, arguments->NativeArgAt(7));
   GET_NATIVE_ARGUMENT(Bool, checked, arguments->NativeArgAt(8));
 
   GET_NATIVE_ARGUMENT(Array, environment, arguments->NativeArgAt(9));
 
   GET_NATIVE_ARGUMENT(String, package_root, arguments->NativeArgAt(10));
   GET_NATIVE_ARGUMENT(Array, packages, arguments->NativeArgAt(11));
 
+  if (Dart::IsRunningPrecompiledCode()) {
+    const Array& args = Array::Handle(Array::New(1));
+    args.SetAt(0, String::Handle(String::New(
+        "Isolate.spawnUri not supported under precompilation")));
+    Exceptions::ThrowByType(Exceptions::kUnsupported, args);
+    UNREACHABLE();
+  }
 
   // Canonicalize the uri with respect to the current isolate.
   const Library& root_lib =
       Library::Handle(isolate->object_store()->root_library());
   char* error = NULL;
   char* canonical_uri = CanonicalizeUri(thread, root_lib, uri, &error);
   if (canonical_uri == NULL) {
     const String& msg = String::Handle(String::New(error));
     ThrowIsolateSpawnException(msg);
   }
@@ skipping 82 matching lines @@
   MessageWriter writer(&data, &allocator, false);
   writer.WriteMessage(msg);
 
   PortMap::PostMessage(new Message(port.Id(),
                                    data, writer.BytesWritten(),
                                    Message::kOOBPriority));
   return Object::null();
 }
 
 }  // namespace dart