Fix debugger stub patching on x64 architecture

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
===================================================================
--- runtime/vm/assembler_x64.cc	(revision 31811)
+++ runtime/vm/assembler_x64.cc	(working copy)
@@ -104,6 +104,32 @@
       object_pool_.Add(Object::null_object(), Heap::kOld);
       patchable_pool_entries_.Add(kNotPatchable);
     }
+
+    // Create fixed object pool entries for debugger stubs.
+    if (StubCode::BreakpointStatic_entry() != NULL) {
+      intptr_t index =
+          FindExternalLabel(&StubCode::BreakpointStaticLabel(), kNotPatchable);
+      ASSERT(index == kBreakpointStaticCPIndex);
+    } else {
+      object_pool_.Add(Object::null_object(), Heap::kOld);
+      patchable_pool_entries_.Add(kNotPatchable);
+    }
+    if (StubCode::BreakpointDynamic_entry() != NULL) {
+      intptr_t index =
+          FindExternalLabel(&StubCode::BreakpointDynamicLabel(), kNotPatchable);
+      ASSERT(index == kBreakpointDynamicCPIndex);
+    } else {
+      object_pool_.Add(Object::null_object(), Heap::kOld);
+      patchable_pool_entries_.Add(kNotPatchable);
+    }
+    if (StubCode::BreakpointRuntime_entry() != NULL) {
+      intptr_t index =
+          FindExternalLabel(&StubCode::BreakpointRuntimeLabel(), kNotPatchable);
+      ASSERT(index == kBreakpointRuntimeCPIndex);
+    } else {
+      object_pool_.Add(Object::null_object(), Heap::kOld);
+      patchable_pool_entries_.Add(kNotPatchable);
+    }
   }
 }