getObject now supports specifying offset,count for strings.

runtime/vm/service/service.md

 # Dart VM Service Protocol 3.1
 
 > Please post feedback to the [observatory-discuss group][discuss-list]
 
 This document describes of _version 3.1_ 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 538 matching lines @@
 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,
+Instance objects with the kinds: String, 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)
 ```
@@ skipping 894 matching lines @@
   //   Int (suitable for passing to int.parse())
   //   String (value may be truncated)
   //   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'.
+  //
+  // New code should use 'length' and 'count' instead.
   bool valueAsStringIsTruncated [optional];
 
-  // The length of a List or the number of associations in a Map.
+  // The length of a List or the number of associations in a Map or the
+  // number of codeunits in a String.
   //
   // Provided for instance kinds:
+  //   String
   //   List
   //   Map
   //   Uint8ClampedList
   //   Uint8List
   //   Uint16List
   //   Uint32List
   //   Uint64List
   //   Int8List
   //   Int16List
   //   Int32List
@@ skipping 48 matching lines @@
   //
   // Provided for the instance kinds:
   //   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'.
+  //
+  // New code should use 'length' and 'count' instead.
   bool valueAsStringIsTruncated [optional];
 
-  // The length of a List or the number of associations in a Map.
+  // The length of a List or the number of associations in a Map or the
+  // number of codeunits in a String.
   //
   // Provided for instance kinds:
+  //   String
   //   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.
+  // The index of the first element or association or codeunit returned.
   // This is only provided when it is non-zero.
   //
   // Provided for instance kinds:
+  //   String
   //   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.
+  // The number of elements or associations or codeunits returned.
   // This is only provided when it is less than length.
   //
   // Provided for instance kinds:
+  //   String
   //   List
   //   Map
   //   Uint8ClampedList
   //   Uint8List
   //   Uint16List
   //   Uint32List
   //   Uint64List
   //   Int8List
   //   Int16List
   //   Int32List
@@ skipping 847 matching lines @@
 }
 ```
 
 ## 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.  Add offset and count parameters to getObject() and offset and count fields to Instance. Added ServiceExtensionAdded event.
-3.1 | Add the getSourceReport RPC.
+3.1 | Add the getSourceReport RPC.  The getObject RPC now accepts offset and count for string objects.  String objects now contain length, offset, and count properties.
 
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss