Observatory:

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

Index: runtime/observatory/lib/src/elements/heap_map.dart
diff --git a/runtime/observatory/lib/src/elements/heap_map.dart b/runtime/observatory/lib/src/elements/heap_map.dart
index bd43b45958835fff3b73413b7bec366b83bbf58b..0abd092241eb06c186835d4f9e0304557e75c28f 100644
--- a/runtime/observatory/lib/src/elements/heap_map.dart
+++ b/runtime/observatory/lib/src/elements/heap_map.dart
@@ -98,6 +98,10 @@ class HeapMapElement extends HtmlElement implements Renderable {
         ..onMouseMove.listen(_handleMouseMove)
         ..onMouseDown.listen(_handleClick);
     }
+
+    // Set hover text to describe the object under the cursor.
+    _canvas.title = _status;
+
     children = [
       navBar([
         new NavTopMenuElement(queue: _r.queue),