- Do not even attempt to spawn an isolate if the local script

runtime/lib/isolate_patch.dart

 // 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.
 
 import "dart:collection" show HashMap;
 import "dart:_internal";
 
 patch class ReceivePort {
   /* patch */ factory ReceivePort() = _ReceivePortImpl;
 
@@ skipping 302 matching lines @@
       // The VM will invoke [_startIsolate] with entryPoint as argument.
       readyPort = new RawReceivePort();
       var packageRoot = null;
       var packageConfig = null;
       if (Isolate._packageSupported()) {
         packageRoot = (await Isolate.packageRoot)?.toString();
         packageConfig = (await Isolate.packageConfig)?.toString();
       }
 
       var script = VMLibraryHooks.platformScript;
-      if (script != null) {
-        if (script.scheme == "package") {
-          script = await Isolate.resolvePackageUri(script);
-        }
+      if (script == null) {
+        // We do not have enough information to support spawning the new
+        // isolate.
+        throw new UnsupportedError("Isolate.spawn");
+      }
+      if (script.scheme == "package") {
+        script = await Isolate.resolvePackageUri(script);
       }
 
       _spawnFunction(readyPort.sendPort, script.toString(), entryPoint, message,
                      paused, errorsAreFatal, onExit, onError,
                      packageRoot, packageConfig);
       return await _spawnCommon(readyPort);
     } catch (e, st) {
       if (readyPort != null) {
         readyPort.close();
       }
@@ skipping 234 matching lines @@
     try {
       return Uri.parse(_getCurrentRootUriStr());
     } catch (e, s) {
       return null;
     }
   }
 
   static String _getCurrentRootUriStr()
       native "Isolate_getCurrentRootUriStr";
 }