Support column-based breakpoints in the VM and Observatory.

runtime/vm/service/service.md

 # Dart VM Service Protocol 2.0
 
 > Please post feedback to the [observatory-discuss group][discuss-list]
 
 This document describes of _version 2.0_ 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 353 matching lines @@
 ReturnType methodName(parameterType parameterName [optional)
 ```
 
 A description of the return types and parameter types is provided
 in the section on [public types](#public-types).
 
 ### addBreakpoint
 
 ```
 Breakpoint addBreakpoint(string isolateId,
-                         string scriptId,
-                         int line)
+                         string scriptId [optional],
+                         string scriptUrl [optional],
+                         int line,
+                         int column)

[rmacnak, 2015/09/03 00:55:19]

int column [optional]

[turnidge, 2015/09/04 18:05:29]

Done.

 ```
 
 The _addBreakpoint_ RPC is used to add a breakpoint at a specific line
 of some script.
 
+The _scriptId_ or _scriptUrl_ parameter is used to specify the target
+script. One of these two parameters must always be provided.
+
+The _line_ parameter is used to specify the target line for the
+breakpoint. If there are multiple possible breakpoints on the target
+line, then the VM will place the breakpoint at the location which
+would execute soonest. If it is not possible to set a breakpoint at
+the target line, the breakpoint will be added at the next possible
+breakpoint location within the same function.
+
+The _column_ parameter may be optionally specified.  This is useful
+for targeting a specific breakpoint on a line with multiple possible
+breakpoints.
+
 If no breakpoint is possible at that line, the _102_ (Cannot add
 breakpoint) error code is returned.
 
 Note that breakpoints are added and removed on a per-isolate basis.
 
 See [Breakpoint](#breakpoint).
 
 ### addBreakpointAtEntry
 
 ```
@@ skipping 1625 matching lines @@
 ```
 
 ## Revision History
 
 version | comments
 ------- | --------
 1.0 draft 1 | initial revision
 
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss