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

lib/isolate/isolate_compiler.dart

Index: lib/isolate/isolate_compiler.dart
diff --git a/lib/isolate/isolate_compiler.dart b/lib/isolate/isolate_compiler.dart
index bc3ab2eadc3cdce736e0dcf099f5b69d31b7ca8c..617d4d3bb229346c396dbb1c660597cd1a6de586 100644
--- a/lib/isolate/isolate_compiler.dart
+++ b/lib/isolate/isolate_compiler.dart
@@ -9,15 +9,12 @@
 
 #source("isolate_api.dart");
 
-class _IsolateFactory {
-
-  factory Isolate2.fromCode(Function topLevelFunction) {
-    throw new NotImplementedException();
-  }
+SendPort _spawnFunction(void topLevelFunction()) {
+  throw new NotImplementedException();
+}
 
-  factory Isolate2.fromUri(String uri) {
-    throw new NotImplementedException();
-  }
+SendPort _spawnUri(String uri) {
+  throw new NotImplementedException();
 }
 
 class _IsolateNatives {