Specify sentinels in the return type for evaluateInFrame.

runtime/vm/service/service.md

 # Dart VM Service Protocol 3.5
 
 > Please post feedback to the [observatory-discuss group][discuss-list]
 
 This document describes of _version 3.5_ 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 476 matching lines @@
 
 If an error occurs while evaluating the expression, an [@Error](#error)
 reference will be returned.
 
 If the expression is evaluated successfully, an [@Instance](#instance)
 reference will be returned.
 
 ### evaluateInFrame
 
 ```
-@Instance|@Error evaluateInFrame(string isolateId,
-                                 int frameIndex,
-                                 string expression)
+@Instance|@Error|Sentinel evaluateInFrame(string isolateId,
+                                          int frameIndex,
+                                          string expression)
 ```
 
 The _evaluateInFrame_ RPC is used to evaluate an expression in the
 context of a particular stack frame. _frameIndex_ is the index of the
 desired [Frame](#frame), with an index of _0_ indicating the top (most
 recent) frame.
 
 If an error occurs while evaluating the expression, an [@Error](#error)
 reference will be returned.
 
@@ skipping 2082 matching lines @@
 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.  The getObject RPC now accepts offset and count for string objects.  String objects now contain length, offset, and count properties.
 3.2 | Isolate objects now include the runnable bit and many debugger related RPCs will return an error if executed on an isolate before it is runnable.
 3.3 | Pause event now indicates if the isolate is paused at an await, yield, or yield* suspension point via the 'atAsyncSuspension' field. Resume command now supports the step parameter 'OverAsyncSuspension'. A Breakpoint added synthetically by an 'OverAsyncSuspension' resume command identifies itself as such via the 'isSyntheticAsyncContinuation' field.
 3.4 | Add the superType and mixin fields to Class. Added new pause event 'None'.
 3.5 | Add the error field to SourceReportRange.  Clarify definition of token position.  Add "Isolate must be paused" error code.
 3.6 (unreleased) | Add 'scopeStartTokenPos', 'scopeEndTokenPos', and 'declarationTokenPos' to BoundVariable. Add 'PausePostRequest' event kind. Add 'Rewind' StepOption. Add error code 107 (isolate cannot resume). Add 'reloadSources' RPC and related error codes.
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss