Check for the type of the the spawned function.

sdk/lib/_internal/js_runtime/lib/isolate_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/isolate_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/isolate_patch.dart b/sdk/lib/_internal/js_runtime/lib/isolate_patch.dart
index 79931ba3039fc1ecb99f22516e6c19d3cb74bccf..ed72e6c09fa64493de08b5803cab910249f3cedf 100644
--- a/sdk/lib/_internal/js_runtime/lib/isolate_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/isolate_patch.dart
@@ -10,6 +10,8 @@ import 'dart:_isolate_helper' show CapabilityImpl,
                                    ReceivePortImpl,
                                    RawReceivePortImpl;
 
+typedef _UnaryFunction(arg);
+
 @patch
 class Isolate {
   static final _currentIsolateCache = IsolateNatives.currentIsolate;
@@ -42,6 +44,11 @@ class Isolate {
                       (onExit != null) ||
                       (onError != null);
     try {
+      // Check for the type of `entryPoint` on the spawning isolate to make
+      // error-handling easier.
+      if (entryPoint is! _UnaryFunction) {
+        throw new ArgumentError(entryPoint);
+      }
       // TODO: Consider passing the errorsAreFatal/onExit/onError values
       //       as arguments to the internal spawnUri instead of setting
       //       them after the isolate has been created.