Change the thread interface in runtime/platform and use it starting all threads

runtime/vm/port_test.cc

Index: runtime/vm/port_test.cc
diff --git a/runtime/vm/port_test.cc b/runtime/vm/port_test.cc
index 3ffc81bed87afbefbc86ad496d315b62b96887ef..d002113f0d04118f1898d2149adcca5620e0a5a5 100644
--- a/runtime/vm/port_test.cc
+++ b/runtime/vm/port_test.cc
@@ -235,8 +235,8 @@ void ThreadedPort_start(uword parameter) {
 TEST_CASE(ThreadedPort) {
   intptr_t local = PortMap::CreatePort();
 
-  Thread* thr = new Thread(ThreadedPort_start, local);
-  EXPECT(thr != NULL);
+  ThreadHandle thread = Thread::Start(ThreadedPort_start, local);
+  EXPECT(thread != Thread::kInvalidThreadHandle);
 
   PortMessage* msg = NextMessage();
   EXPECT_EQ(local, msg->dest_port());