Insert debug step check at the beginning of a function.

runtime/observatory/tests/service/debugging_test.dart

 // Copyright (c) 2015, 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.
 // VMOptions=--compile_all --error_on_bad_type --error_on_bad_override
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 import 'dart:async';
 
@@ skipping 169 matching lines @@
   ServiceFunction function = isolate.rootLibrary.functions.firstWhere(
       (f) => f.name == 'periodicTask');
   expect(function, isNotNull);
 
   // Add the breakpoint at function entry
   var result = await isolate.addBreakpointAtEntry(function);
   expect(result is Breakpoint, isTrue);
   Breakpoint bpt = result;
   expect(bpt.type, equals('Breakpoint'));
   expect(bpt.location.script.name, equals('debugging_test.dart'));
-  expect(bpt.location.script.tokenToLine(bpt.location.tokenPos), equals(14));
+  expect(bpt.location.script.tokenToLine(bpt.location.tokenPos), equals(13));
   expect(isolate.breakpoints.length, equals(1));
 
   await completer.future;  // Wait for breakpoint events.
 },
 
-// We are now at line 14.
+// We are now at line 13.
 (Isolate isolate) async {
   ServiceMap stack = await isolate.getStack();
   expect(stack.type, equals('Stack'));
   expect(stack['frames'].length, greaterThanOrEqualTo(1));
 
   Script script = stack['frames'][0].location.script;
   expect(script.name,endsWith('debugging_test.dart'));
-  expect(script.tokenToLine(stack['frames'][0].location.tokenPos), equals(14));
+  expect(script.tokenToLine(stack['frames'][0].location.tokenPos), equals(13));
 },
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: startTimer);