Report the reason for a garbage collection when verbose GC is enabled.

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index d3bf0703bfc80822538a5da47c497178ab930e1e..0c0a444dabc34ebfbfe441fd718079d3daab81fd 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -38,6 +38,16 @@ class Heap {
     kInvokeApiCallbacks
   };
 
+  enum GCReason {
+    kNewSpace,
+    kPromotionFailure,
+    kOldSpace,
+    kCodeSpace,
+    kFull,
+    kGCAtAlloc,
+    kGCTestCase,
+  };
+
   // Default allocation sizes in MB for the old gen and code heaps.
   static const intptr_t kHeapSizeInMB = 512;
   static const intptr_t kCodeHeapSizeInMB = 12;
@@ -144,6 +154,8 @@ class Heap {
   // Generates a profile of the current and VM isolate heaps.
   void Profile(Dart_HeapProfileWriteCallback callback, void* stream) const;
 
+  static const char* GCReasonToString(GCReason gc_reason);
+
  private:
   Heap();