Switch to using SourceLocation universally in service protocol.

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 50 matching lines @@
         - [Isolate](#isolate)
         - [Library](#library)
         - [LibraryDependency](#librarydependency)
         - [ListElement](#listelement)
         - [Message](#message)
         - [Null](#null)
         - [Object](#object)
         - [Sentinel](#sentinel)
         - [SentinelKind](#sentinelkind)
         - [Script](#script)
+        - [SourceLocation](#sourcelocation)
         - [Stack](#stack)
         - [StepOption](#stepoption)
         - [Success](#success)
         - [TypeArguments](#typearguments)
         - [Response](#response)
         - [Version](#version)
         - [VM](#vm)
 - [Revision History](#revision-history)
 
 ## RPCs, Requests, and Responses
@@ skipping 652 matching lines @@
 
 If the variable has been optimized out by the compiler, the _value_
 will be the _OptimizedOut_ [Sentinel](#sentinel).
 
 ### Breakpoint
 
 ```
 class Breakpoint extends Response {
   int breakpointNumber;
   bool resolved;
-  @Script script;
-  int tokenPos;
+  SourceLocation location;
 }
 ```
 
 A _Breakpoint_ describes a debugger breakpoint.
 
 ### Class
 
 ```
 class @Class extends @Object {
   // The name of this class.
@@ skipping 22 matching lines @@
 
   // Is this class implemented?
   bool implemented;
 
   // Is this a vm patch class?
   bool patch;
 
   // The library which contains this class.
   @Library library;
 
-  // The script which defines this class.  May be missing for some
-  // classes.
-  @Script script;
+  // The location of this class in the source code.
+  SourceLocation location [optional];
 
   // The superclass of this class, if any.
   @Class super [optional];
 
   // A list of interface types for this class.
   @Type[] interfaces;
 
   // A list of fields in this class.  Does not include fields from
   // superclasses.
   @Field[] fields;
@@ skipping 247 matching lines @@
 
   // Is this field final?
   bool final;
 
   // Is this field static?
   bool static;
 
   // The value of this field, if the field is static.
   @Instance staticValue [optional];
 
-  // The script containing this feild.
-  @Script script [optional];
-
-  // The token position of this field.
-  int tokenPos [optional];
+  // The location of this field in the source code.
+  SourceLocation location [optional];
 }
 ```
 
 A _Field_ provides information about a Dart language field or
 variable.
 
 
 ### Flag
 
 ```
@@ skipping 23 matching lines @@
   // A list of all flags which have been modified by the user.
   modifiedFlags []Flag
 }
 ```
 
 A _FlagList_ represents the complete set of VM command line flags.
 
 ### Frame
 
 ```
-class Frame {
+class Frame extends Response {
   int index;
   @Function function;
   @Code code;
   @Script script;
   int tokenPos;
   BoundVariable[] vars;
 }
 ```
 
 ### Function
@@ skipping 22 matching lines @@
 ```
 // A Dart language function.
 class Function extends Object {
   // The name of this function.
   string name;
 
   // The owner of this field, which can be a Library, Class, or a
   // Function.
   @Library|@Class|@Function owner;
 
-  // The script containing this function.
-  @Script script [optional];
-
-  // The first token position of this function.
-  int tokenPos [optional];
-
-  // The last token position of this function.
-  int endTokenPos [optional];
+  // The location of this function in the source code.
+  SourceLocation location [optional];
 
   // The compiled code associated with this function.
   @Code code [optional];
 }
 ```
 
 A _Function_ represents a Dart language function.
 
 ### Instance
 
@@ skipping 376 matching lines @@
 ```
 class MapAssociation {
   @Instance|Sentinel key;
   @Instance|Sentinel value;
 }
 ```
 
 ### Message
 
 ```
-class Message {
+class Message extends Response {
   int index;
   string name;
   string messageObjectId;
   int size;
-  @Function handlerFunction [optional];
-  @Script handleScript [optional];
-  int handlerTokenPos [optional];
+  @Function handler [optional];
+  SourceLocation location [optional];
 }
 ```
 
 ### Null
 
 ```
 class @Null extends @Instance {
   // Always 'null'.
   string valueAsString;
 }
@@ skipping 128 matching lines @@
 > [[1, 100, 5, 101, 8],[2, 102, 7]]
 
 ...encodes the mapping:
 
 tokenPos | line | column
 -------- | ---- | ------
 100 | 1 | 5
 101 | 1 | 8
 102 | 2 | 7
 
+### SourceLocation
+
+```
+class SourceLocation extends Response {
+  // The script contaiinging the source location.
+  @Script script;
+
+  // The first token of the location.
+  int tokenPos;
+
+  // The last token of the location if this is a range.
+  int endTokenPos [optional];
+}
+```
+
+The _SourceLocation_ class is used to designate a position or range in
+some script.
+
 ### Stack
 
 ```
 class Stack {
   Frame[] frames;
   Message[] messages;
 }
 ```
 
 ### StepOption
@@ skipping 99 matching lines @@
   // A list of isolates running in the VM.
   @Isolate[] isolates
 }
 ```
 
 ## Revision History
 
 version | comments
 ------- | --------
 0.0 | draft