Fix http://dartbug.com/22115

tests/isolate/spawn_uri_exported_main_test.dart

+import "dart:async";
+import "dart:isolate";
+import "package:expect/expect.dart";
+
+main() {
+  print("Spawning isolate.");
+  var t = new Timer(new Duration(seconds:5), () {
+        Expect.fail("Isolate was not spawned successfully.");
+  });
+  var rp = new RawReceivePort();
+  rp.handler = (msg) {
+    print("Spawned main called.");
+    Expect.equals(msg, 50);
+        rp.close();
+  };
+  Isolate.spawnUri(Uri.parse("spawn_uri_exported_main.dart"), null, rp.sendPort).then((_) {
+    print("Loaded");
+        t.cancel();
+  });
+}