Reduce GC from external allocation.

runtime/vm/dart_api_impl_test.cc

Index: runtime/vm/dart_api_impl_test.cc
===================================================================
--- runtime/vm/dart_api_impl_test.cc	(revision 34481)
+++ runtime/vm/dart_api_impl_test.cc	(working copy)
@@ -2306,24 +2306,30 @@
 
 
 TEST_CASE(WeakPersistentHandleExternalAllocationSizeOversized) {
+  Dart_Isolate isolate = reinterpret_cast<Dart_Isolate>(Isolate::Current());
   Heap* heap = Isolate::Current()->heap();
   Dart_WeakPersistentHandle weak1 = NULL;
-  const intptr_t kWeak1ExternalSize = 100 * MB;
+  // Large enough to exceed any new space limit. Not actually allocated.
+  const intptr_t kWeak1ExternalSize = 500 * MB;
   {
     Dart_EnterScope();
     Dart_Handle obj = NewString("weakly referenced string");
     EXPECT_VALID(obj);
+    // Triggers a scavenge immediately, since kWeak1ExternalSize is above limit.
     weak1 = Dart_NewWeakPersistentHandle(obj,
                                          NULL,
                                          kWeak1ExternalSize,
                                          NopCallback);
     EXPECT_VALID(AsHandle(weak1));
-    // While new space is "full" of external data, any allocation will
-    // trigger GC, so after two of them, obj should be promoted.
-    Dart_Handle trigger1 = NewString("trigger1");
-    EXPECT_VALID(trigger1);
-    Dart_Handle trigger2 = NewString("trigger2");
-    EXPECT_VALID(trigger2);
+    // ... but the object is still alive and not yet promoted, so external size
+    // in new space is still above the limit. Thus, even the following tiny
+    // external allocation will trigger another scavenge.
+    Dart_WeakPersistentHandle trigger =
+      Dart_NewWeakPersistentHandle(obj, NULL, 1, NopCallback);
+    EXPECT_VALID(AsHandle(trigger));
+    Dart_DeleteWeakPersistentHandle(isolate, trigger);
+    // After the two scavenges above, 'obj' should now be promoted, hence its
+    // external size charged to old space.
     {
       DARTSCOPE(Isolate::Current());
       String& handle = String::Handle();
@@ -2334,7 +2340,6 @@
     EXPECT(heap->ExternalInWords(Heap::kOld) == kWeak1ExternalSize / kWordSize);
     Dart_ExitScope();
   }
-  Dart_Isolate isolate = reinterpret_cast<Dart_Isolate>(Isolate::Current());
   Dart_DeleteWeakPersistentHandle(isolate, weak1);
   Isolate::Current()->heap()->CollectGarbage(Heap::kOld);
   EXPECT(heap->ExternalInWords(Heap::kOld) == 0);