Use offset and count to request slices of lists, maps, and typed_data.

runtime/vm/service/service.md

 # Dart VM Service Protocol 3.0
 
 > Please post feedback to the [observatory-discuss group][discuss-list]
 
 This document describes of _version 3.0_ 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,
@@ skipping 508 matching lines @@
 
 If _isolateId_ refers to an isolate which has exited, then the
 _Collected_ [Sentinel](#sentinel) is returned.
 
 See [Isolate](#isolate).
 
 ### getObject
 
 ```
 Object|Sentinel getObject(string isolateId,
-                          string objectId)
+                          string objectId,
+                          int offset [optional])
+                          int count [optional])
 ```
 
 The _getObject_ RPC is used to lookup an _object_ from some isolate by
 its _id_.
 
 If _objectId_ is a temporary id which has expired, then then _Expired_
 [Sentinel](#sentinel) is returned.
 
 If _objectId_ refers to a heap object which has been collected by the VM's
 garbage collector, then the _Collected_ [Sentinel](#sentinel) is
 returned.
 
 If _objectId_ refers to a non-heap object which has been deleted, then
 the _Collected_ [Sentinel](#sentinel) is returned.
 
 If the object handle has not expired and the object has not been
 collected, then an [Object](#object) will be returned.
 
+The _offset_ and _count_ parameters are used to request subranges of
+Instance objects with the kinds: List, Map, Uint8ClampedList,
+Uint8List, Uint16List, Uint32List, Uint64List, Int8List, Int16List,
+Int32List, Int64List, Flooat32List, Float64List, Inst32x3List,
+Float32x4List, and Float64x2List.  These parameters are otherwise
+ignored.
+
 ### getStack
 
 ```
 Stack getStack(string isolateId)
 ```
 
 The _getStack_ RPC is used to retrieve the current execution stack and
 message queue for an isolate. The isolate does not need to be paused.
 
 See [Stack](#stack).
@@ skipping 780 matching lines @@
   //   Float32x4
   //   Float64x2
   //   Int32x4
   //   StackTrace
   string valueAsString [optional];
 
   // The valueAsString for String references may be truncated. If so,
   // this property is added with the value 'true'.
   bool valueAsStringIsTruncated [optional];
 
-  // The length of a List instance.
+  // The length of a List or the number of associations in a Map.
   //
   // Provided for instance kinds:
   //   List
   //   Map
   //   Uint8ClampedList
   //   Uint8List
   //   Uint16List
   //   Uint32List
   //   Uint64List
   //   Int8List
@@ skipping 52 matching lines @@
   //   Bool (true or false)
   //   Double (suitable for passing to Double.parse())
   //   Int (suitable for passing to int.parse())
   //   String (value may be truncated)
   string valueAsString [optional];
 
   // The valueAsString for String references may be truncated. If so,
   // this property is added with the value 'true'.
   bool valueAsStringIsTruncated [optional];
 
-  // The length of a List instance.
+  // The length of a List or the number of associations in a Map.
   //
   // Provided for instance kinds:
   //   List
   //   Map
   //   Uint8ClampedList
   //   Uint8List
   //   Uint16List
   //   Uint32List
   //   Uint64List
   //   Int8List
   //   Int16List
   //   Int32List
   //   Int64List
   //   Float32List
   //   Float64List
   //   Int32x4List
   //   Float32x4List
   //   Float64x2List
   int length [optional];
 
+  // The index of the first element or association returned.
+  // This is only provided when it is non-zero.
+  //
+  // Provided for instance kinds:
+  //   List
+  //   Map
+  //   Uint8ClampedList
+  //   Uint8List
+  //   Uint16List
+  //   Uint32List
+  //   Uint64List
+  //   Int8List
+  //   Int16List
+  //   Int32List
+  //   Int64List
+  //   Float32List
+  //   Float64List
+  //   Int32x4List
+  //   Float32x4List
+  //   Float64x2List
+  int offset [optional];
+
+  // The number of elements or associations returned.
+  // This is only provided when it is less than length.
+  //
+  // Provided for instance kinds:
+  //   List
+  //   Map
+  //   Uint8ClampedList
+  //   Uint8List
+  //   Uint16List
+  //   Uint32List
+  //   Uint64List
+  //   Int8List
+  //   Int16List
+  //   Int32List
+  //   Int64List
+  //   Float32List
+  //   Float64List
+  //   Int32x4List
+  //   Float32x4List
+  //   Float64x2List
+  int count [optional];
+
   // The name of a Type instance.
   //
   // Provided for instance kinds:
   //   Type
   string name [optional];
 
   // The corresponding Class if this Type is canonical.
   //
   // Provided for instance kinds:
   //   Type
@@ skipping 709 matching lines @@
   @Isolate[] isolates;
 }
 ```
 
 ## Revision History
 
 version | comments
 ------- | --------
 1.0 | initial revision
 2.0 | Describe protocol version 2.0.
-3.0 | Describe protocol version 3.0.  Added UnresolvedSourceLocation.  Added Sentinel return to getIsolate.  Add AddedBreakpointWithScriptUri.  Removed Isolate.entry. The type of VM.pid was changed from string to int.  Added VMUpdate events.
+3.0 | Describe protocol version 3.0.  Added UnresolvedSourceLocation.  Added Sentinel return to getIsolate.  Add AddedBreakpointWithScriptUri.  Removed Isolate.entry. The type of VM.pid was changed from string to int.  Added VMUpdate events.  Add offset and count parameters to getObject() and offset and count fields to Instance.
 
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss