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

runtime/vm/service/service.md

 # Dart VM Service Protocol 0.0
 
 This document describes _version 0.0_ of the Dart VM Service Protocol.
 This protocol is used to communicate with a running Dart Virtual
 Machine.
 
 To use the Service Protocol, start the VM with the *--observe* flag.
 The VM will start a webserver which services protocol requests via WebSocket.
 It is possible to make HTTP (non-WebSocket) requests,
 but this does not allow access to VM _events_ and is not documented
@@ skipping 1249 matching lines @@
 
   // The fields of this Instance.
   BoundField fields [optional];
 
   // The elements of a List instance.
   //
   // Provided for instance kinds:
   //   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:
   //   Closure
   @Function closureFunction [optional];
 
   // The context associated with a Closure instance.
   //
   // Provided for instance kinds:
   //   Closure
@@ skipping 244 matching lines @@
 
 ### ListElement
 
 ```
 class ListElement {
   int index;
   @Instance|Sentinel value;
 }
 ```
 
+### MapAssociation
+
+```
+class MapAssociation {
+  @Instance|Sentinel key;
+  @Instance|Sentinel value;
+}
+```
+
 ### Message
 
 ```
 class Message {
   int index;
   string name;
   string messageObjectId;
   int size;
   @Function handlerFunction [optional];
   @Script handleScript [optional];
@@ skipping 258 matching lines @@
   // A list of isolates running in the VM.
   @Isolate[] isolates
 }
 ```
 
 ## Revision History
 
 version | comments
 ------- | --------
 0.0 | draft