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

runtime/vm/scavenger.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_SCAVENGER_H_
 #define VM_SCAVENGER_H_
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/flags.h"
@@ skipping 25 matching lines @@
     // least unless it is debugging code. This might need to be relaxed later,
     // but currently it helps prevent dumb bugs.
     ASSERT(!from_->Contains(addr));
     return to_->Contains(addr);
   }
 
   uword TryAllocate(intptr_t size) {
     ASSERT(Utils::IsAligned(size, kObjectAlignment));
 #if defined(DEBUG)
     if (FLAG_gc_at_alloc && !scavenging_) {
-      Scavenge();
+      Scavenge("debugging");
     }
 #endif
     uword result = top_;
     intptr_t remaining = end_ - top_;
     if (remaining < size) {
       return 0;
     }
     ASSERT(to_->Contains(result));
     ASSERT((result & kObjectAlignmentMask) == object_alignment_);
 
     top_ += size;
     ASSERT(to_->Contains(top_) || (top_ == to_->end()));
     return result;
   }
 
   // Collect the garbage in this scavenger.
-  void Scavenge();
-  void Scavenge(bool invoke_api_callbacks);
+  void Scavenge(const char* gc_reason);
+  void Scavenge(bool invoke_api_callbacks, const char* gc_reason);
 
   // Accessors to generate code for inlined allocation.
   uword* TopAddress() { return &top_; }
   uword* EndAddress() { return &end_; }
   static intptr_t top_offset() { return OFFSET_OF(Scavenger, top_); }
   static intptr_t end_offset() { return OFFSET_OF(Scavenger, end_); }
 
   intptr_t in_use() const { return (top_ - FirstObjectStart()); }
   intptr_t capacity() const { return space_->size(); }
 
@@ skipping 80 matching lines @@
 
   friend class ScavengerVisitor;
   friend class ScavengerWeakVisitor;
 
   DISALLOW_COPY_AND_ASSIGN(Scavenger);
 };
 
 }  // namespace dart
 
 #endif  // VM_SCAVENGER_H_