Add 'startPaused' option to Isolate.spawn*.

runtime/lib/isolate_patch.dart

Index: runtime/lib/isolate_patch.dart
diff --git a/runtime/lib/isolate_patch.dart b/runtime/lib/isolate_patch.dart
index a5955a94bb4e1fa853c7f785852cbed467672994..1c9fa4a76a1505363d3838d37c11fb23281037b9 100644
--- a/runtime/lib/isolate_patch.dart
+++ b/runtime/lib/isolate_patch.dart
@@ -227,7 +227,7 @@ void _startIsolate(Function entryPoint, bool isSpawnUri) {
 
 patch class Isolate {
   /* patch */ static Future<Isolate> spawn(
-      void entryPoint(message), var message) {
+      void entryPoint(message), var message, { bool startPaused: false }) {
     try {
       // The VM will invoke [_startIsolate] with entryPoint as argument.
       SendPort controlPort = _spawnFunction(entryPoint);
@@ -246,7 +246,7 @@ patch class Isolate {
   }
 
   /* patch */ static Future<Isolate> spawnUri(
-      Uri uri, List<String> args, var message) {
+      Uri uri, List<String> args, var message, { bool startPaused: false }) {
     try {
       // The VM will invoke [_startIsolate] and not `main`.
       SendPort controlPort = _spawnUri(uri.toString());