Support column-based breakpoints in the VM and Observatory.

runtime/vm/service/service.md

Index: runtime/vm/service/service.md
diff --git a/runtime/vm/service/service.md b/runtime/vm/service/service.md
index c18f390d9bc59962a9686c5b0dea7e7e7e99e268..eff368c114ebc42e60258fa500cccda0a5b0f86a 100644
--- a/runtime/vm/service/service.md
+++ b/runtime/vm/service/service.md
@@ -371,13 +371,29 @@ in the section on [public types](#public-types).
 
 ```
 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.