Only reload libraries when they may have been modified.

runtime/bin/gen_snapshot.cc

Index: runtime/bin/gen_snapshot.cc
diff --git a/runtime/bin/gen_snapshot.cc b/runtime/bin/gen_snapshot.cc
index d75a3476136b2593a94b1de560a440ee58488e15..8654e2842c82ee88019b30cc4b15872441c483db 100644
--- a/runtime/bin/gen_snapshot.cc
+++ b/runtime/bin/gen_snapshot.cc
@@ -491,9 +491,9 @@ static Dart_Handle LoadSnapshotCreationScript(const char* script_name) {
     return source;
   }
   if (IsSnapshottingForPrecompilation()) {
-    return Dart_LoadScript(resolved_uri, source, 0, 0);
+    return Dart_LoadScript(resolved_uri, Dart_Null(), source, 0, 0);
   } else {
-    return Dart_LoadLibrary(resolved_uri, source, 0, 0);
+    return Dart_LoadLibrary(resolved_uri, Dart_Null(), source, 0, 0);
   }
 }
 
@@ -558,7 +558,7 @@ static Dart_Handle CreateSnapshotLibraryTagHandler(Dart_LibraryTag tag,
   if (libraryBuiltinId != Builtin::kInvalidLibrary) {
     // Special case for parting sources of a builtin library.
     if (tag == Dart_kSourceTag) {
-      return Dart_LoadSource(library, url,
+      return Dart_LoadSource(library, url, Dart_Null(),
           Builtin::PartSource(libraryBuiltinId, url_string), 0, 0);
     }
     ASSERT(tag == Dart_kImportTag);
@@ -579,10 +579,10 @@ static Dart_Handle CreateSnapshotLibraryTagHandler(Dart_LibraryTag tag,
     return source;
   }
   if (tag == Dart_kImportTag) {
-    return Dart_LoadLibrary(url, source, 0, 0);
+    return Dart_LoadLibrary(url, Dart_Null(), source, 0, 0);
   } else {
     ASSERT(tag == Dart_kSourceTag);
-    return Dart_LoadSource(library, url, source, 0, 0);
+    return Dart_LoadSource(library, url, Dart_Null(), source, 0, 0);
   }
 }
 
@@ -721,7 +721,8 @@ static void SetupStubNativeResolver(size_t lib_index,
              lib_index);
     Dart_Handle script_url = Dart_NewStringFromCString(load_buffer);
     free(load_buffer);
-    Dart_Handle loaded = Dart_LoadLibrary(script_url, script_handle, 0, 0);
+    Dart_Handle loaded = Dart_LoadLibrary(script_url, Dart_Null(),
+                                          script_handle, 0, 0);
     DART_CHECK_VALID(loaded);
 
     // Do a fresh lookup