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

tests/isolate/src/APIv2_spawnUriBrowserTest.dart

Index: tests/isolate/src/APIv2_spawnUriBrowserTest.dart
diff --git a/tests/isolate/src/APIv2_spawnUriBrowserTest.dart b/tests/isolate/src/APIv2_spawnUriBrowserTest.dart
index 5389885ac92d43f711bbce9eb1b20825b955164b..b79b082e767717c4f209233b630689c2f988f0cb 100644
--- a/tests/isolate/src/APIv2_spawnUriBrowserTest.dart
+++ b/tests/isolate/src/APIv2_spawnUriBrowserTest.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());
   });
 }