Add --break-at-isolate-spawn to help with isolate debugging.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index a90137d56724e31b06c859c90c39b4e2f7367487..74d0394a57ab2dae0801233c96f34af7cf807a54 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -43,6 +43,9 @@ DEFINE_FLAG(bool, pause_isolates_on_start, false,
             "Pause isolates before starting.");
 DEFINE_FLAG(bool, pause_isolates_on_exit, false,
             "Pause isolates exiting.");
+DEFINE_FLAG(bool, break_at_isolate_spawn, false,
+            "Insert a one-time breakpoint at the entrypoint for all spawned "
+            "isolates");
 
 
 // Quick access to the locally defined isolate() method.
@@ -836,6 +839,14 @@ static bool RunIsolate(uword parameter) {
     func ^= result.raw();
     func = func.ImplicitClosureFunction();
 
+    // TODO(turnidge): Currently we need a way to force a one-time
+    // breakpoint for all spawned isolates to support isolate
+    // debugging.  Remove this once the vmservice becomes the standard
+    // way to debug.
+    if (FLAG_break_at_isolate_spawn) {
+      isolate->debugger()->OneTimeBreakAtEntry(func);
+    }
+
     const Array& capabilities = Array::Handle(Array::New(2));
     Capability& capability = Capability::Handle();
     capability = Capability::New(isolate->pause_capability());