[vm service] Collect garbage before getting native allocation samples

runtime/observatory/lib/src/elements/native_memory_profiler.dart

Index: runtime/observatory/lib/src/elements/native_memory_profiler.dart
diff --git a/runtime/observatory/lib/src/elements/native_memory_profiler.dart b/runtime/observatory/lib/src/elements/native_memory_profiler.dart
index 688f4fd9928b54e006b0ce00d79aecc7ff1eb712..2820afe90ab423032a7a402ad86d67a8c3394bcf 100644
--- a/runtime/observatory/lib/src/elements/native_memory_profiler.dart
+++ b/runtime/observatory/lib/src/elements/native_memory_profiler.dart
@@ -143,6 +143,9 @@ class NativeMemoryProfileElement extends HtmlElement implements Renderable {
   }
 
   Future _request({bool forceFetch: false}) async {
+    for (Isolate isolate in vm.isolates) {
+      await isolate.invokeRpc("_collectAllGarbage", {});
+    }
     _progress = null;
     _progressStream = _profiles.get(_vm, _tag, forceFetch: forceFetch);
     _r.dirty();