Set the loaded_ flag in RawLibrary when it is completely loaded. This

vm/raw_object.h

Index: vm/raw_object.h
===================================================================
--- vm/raw_object.h	(revision 1579)
+++ vm/raw_object.h	(working copy)
@@ -438,6 +438,13 @@
 
 
 class RawLibrary : public RawObject {
+  enum LibraryState {
+    kAllocated,       // Initial state.
+    kLoadInProgress,  // Library is in the process of being loaded.
+    kLoaded,          // Library is loaded.
+    kLoadError,       // Error occurred during load of the Library.
+  };
+
   RAW_HEAP_OBJECT_IMPLEMENTATION(Library);
 
   RawObject** from() { return reinterpret_cast<RawObject**>(&ptr()->name_); }
@@ -460,7 +467,7 @@
   intptr_t num_anonymous_;       // Number of entries in anonymous_classes_.
   Dart_NativeEntryResolver native_entry_resolver_;  // Resolves natives.
   bool corelib_imported_;
-  bool loaded_;
+  int8_t load_state_;            // Of type LibraryState.
 };