Fixed missing method referenced when MallocHooks is unsupported.

runtime/vm/malloc_hooks_unsupported.cc

 // Copyright (c) 2017, 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.
 
 #include "platform/globals.h"
 
 #if !defined(DART_USE_TCMALLOC) || defined(PRODUCT) ||                         \
     defined(TARGET_ARCH_DBC) || defined(HOST_OS_FUCHSIA)
 
 #include "vm/malloc_hooks.h"
@@ skipping 33 matching lines @@
 bool MallocHooks::Active() {
   return false;
 }
 
 
 void MallocHooks::PrintToJSONObject(JSONObject* jsobj) {
   // Do nothing.
 }
 
 
+Sample* MallocHooks::GetSample(const void* ptr) {
+  return NULL;
+}
+
+
 intptr_t MallocHooks::allocation_count() {
   return 0;
 }
 
 
 intptr_t MallocHooks::heap_allocated_memory_in_bytes() {
   return 0;
 }
 
 }  // namespace dart
 
 #endif  // !defined(DART_USE_TCMALLOC) || defined(PRODUCT) ||
         // defined(TARGET_ARCH_DBC) || defined(HOST_OS_FUCHSIA)