Fix linux build

runtime/vm/debugger.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/debugger.h"
 
 #include "vm/code_index_table.h"
 #include "vm/code_patcher.h"
 #include "vm/compiler.h"
 #include "vm/flags.h"
@@ skipping 116 matching lines @@
     const Script& script = Script::Handle(SourceScript());
     intptr_t ignore_column;
     script.GetTokenLocation(TokenIndex(), &line_number_, &ignore_column);
   }
   return line_number_;
 }
 
 
 RawArray* ActivationFrame::Variables() {
   UNIMPLEMENTED();
+  return NULL;
 }
 
 
 RawInstance* ActivationFrame::Value(const String& variable_name) {
   UNIMPLEMENTED();
+  return NULL;
 }
 
 
 const char* ActivationFrame::ToCString() {
   const char* kFormat = "Function: '%s%s%s' url: '%s' line: %d";
 
   Function& func = Function::Handle(DartFunction());
   String& func_name = String::Handle(func.name());
   Class& func_class = Class::Handle(func.owner());
   String& class_name = String::Handle(func_class.Name());
@@ skipping 201 matching lines @@
 
 
 void Debugger::AddBreakpoint(Breakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(this->breakpoints_);
   this->breakpoints_ = bpt;
 }
 
 
 }  // namespace dart