Add breakpoints and single-stepping to Observatory.

runtime/vm/debugger.h

Index: runtime/vm/debugger.h
diff --git a/runtime/vm/debugger.h b/runtime/vm/debugger.h
index 51b1ddb84083a8eddfccfb8f4c62883692de7f4b..90dd1c0a28ab71d97159dea0739d7fea1a3d129f 100644
--- a/runtime/vm/debugger.h
+++ b/runtime/vm/debugger.h
@@ -350,6 +350,8 @@ class Debugger {
 
   // Set breakpoint at closest location to function entry.
   SourceBreakpoint* SetBreakpointAtEntry(const Function& target_function);
+
+  // TODO(turnidge): script_url may no longer be specific enough.
   SourceBreakpoint* SetBreakpointAtLine(const String& script_url,
                                         intptr_t line_number);
   void OneTimeBreakAtEntry(const Function& target_function);