A little bit more of the threads implementation.

The most important change here is understanding that Thread's id_ field
was unrelated to java.lang.Thread's id field.

Change-Id: I832b92145332e1ded63a7824033dae684eeacf28
diff --git a/src/object.cc b/src/object.cc
index 8b0c976..4384a24 100644
--- a/src/object.cc
+++ b/src/object.cc
@@ -50,7 +50,7 @@
 }
 
 Class* Field::GetType() const {
-  DCHECK(Runtime::Current() != NULL)
+  DCHECK(Runtime::Current()->IsStarted())
       << "Can't call GetType without an initialized runtime";
   // Do full linkage (which sets dex cache value to speed next call)
   return Runtime::Current()->GetClassLinker()->ResolveType(GetTypeIdx(), this);
@@ -498,7 +498,7 @@
 
 void Class::SetStatus(Status new_status) {
   CHECK(new_status > GetStatus() || new_status == kStatusError ||
-        Runtime::Current() == NULL);  // no runtime implies we're not initialized
+      !Runtime::Current()->IsStarted());
   CHECK(sizeof(Status) == sizeof(uint32_t));
   return SetField32(OFFSET_OF_OBJECT_MEMBER(Class, status_),
                     new_status, false);