isolates: remove Isolate2 interface, add top-level spawn functions, more comments

tests/isolate/src/APIv2_spawnUriBrowserNegativeTest.dart

Index: tests/isolate/src/APIv2_spawnUriBrowserNegativeTest.dart
diff --git a/tests/isolate/src/APIv2_spawnUriBrowserNegativeTest.dart b/tests/isolate/src/APIv2_spawnUriBrowserNegativeTest.dart
index d801d45a92117cc75e8d3ae48263a3b4ef9b0408..5e235ade2c60bf0aa3c05f12956fd73e400d39d2 100644
--- a/tests/isolate/src/APIv2_spawnUriBrowserNegativeTest.dart
+++ b/tests/isolate/src/APIv2_spawnUriBrowserNegativeTest.dart
@@ -21,7 +21,7 @@ main() {
     });
 
     // TODO(eub): make this work for non-JS targets.
-    Isolate2 c = new Isolate2.fromUri('APIv2_spawnUriChildIsolate.js');
-    c.sendPort.send('hi', port.toSendPort());
+    SendPort s = spawnUri('APIv2_spawnUriChildIsolate.js');
+    s.send('hi', port.toSendPort());
   });
 }