Second version of support for specifying an unhandled exception callback

runtime/tests/vm/dart/isolate_unhandled_exception_uri_test.dart

+import 'dart:isolate';
+import 'dart:io';
+
+// Tests that isolate code in another script keeps message handling working
+// after throwing an unhandled exception, if it has a function called
+// unhandledExceptionCallback that returns true (continue handling).
+
+// Note: this test will hang if an uncaught exception isn't handled,
+// either by an error in the callback or it returning false.
+
+void main() {
+  var isolate_port = spawnUri('isolate_unhandled_exception_uri_helper.dart');
+
+  // Send a message that will cause an ignorable exception to be thrown.
+  Future f = isolate_port.call('throw exception');
+  f.onComplete((future) {
+    // Exception wasn't ignored as it was supposed to be.
+    print('failed handling exception');
+    exit(1);
+  });
+
+  // Verify that isolate can still handle messages.
+  isolate_port.call('hi').onComplete((future) {
+    if (future.exception != null) {
+      print('unhandled exception: ${future.exception}');
+      exit(1);
+    }
+    if (future.value != 'hello') {
+      print('unexpected response: ${future.value}');
+      exit(1);
+    }
+    exit(0);
+  });
+}