Provide a logical view of VM-internal maps in Observatory.

runtime/vm/service/service.md

Index: runtime/vm/service/service.md
diff --git a/runtime/vm/service/service.md b/runtime/vm/service/service.md
index 005a771a6052bfaa196ec8320ad1a6ee59384f4c..4fb44ce1ffeab92f3dbdbba839cc5d7ab1448c4c 100644
--- a/runtime/vm/service/service.md
+++ b/runtime/vm/service/service.md
@@ -1267,6 +1267,12 @@ class Instance extends Object {
   //   List
   ListElement[] elements [optional];
 
+  // The elements of a List instance.
+  //
+  // Provided for instance kinds:
+  //   Map
+  MapAssociation[] associations [optional];
+
   // The function associated with a Closure instance.
   //
   // Provided for instance kinds:
@@ -1531,6 +1537,15 @@ class ListElement {
 }
 ```
 
+### MapAssociation
+
+```
+class MapAssociation {
+  @Instance|Sentinel key;
+  @Instance|Sentinel value;
+}
+```
+
 ### Message
 
 ```