Insert debug step check at the beginning of a function.

runtime/observatory/tests/service/eval_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';
 
 int globalVar = 100;
 
 class MyClass {
   static int staticVar = 1000;
 
   static void printValue(int value) {
-    print(value);   // line 16
+    print(value);   // line 17
   }
 }
 
 void testFunction() {
   int i = 0;
   while (true) {
     if (++i % 100000000 == 0) {
       MyClass.printValue(10000);
     }
   }
@@ skipping 12 matching lines @@
           if (event.kind == ServiceEvent.kPauseBreakpoint) {
             print('Breakpoint reached');
             subscription.cancel();
             completer.complete();
           }
         });
       });
 
       // Add the breakpoint.
       var script = isolate.rootLibrary.scripts[0];
-      var line = 16;
+      var line = 17;
       return isolate.addBreakpoint(script, line).then((ServiceObject bpt) {
           return completer.future;  // Wait for breakpoint reached.
       });
     });
 },
 
 // Evaluate against library, class, and instance.
 (Isolate isolate) {
   return isolate.getStack().then((ServiceMap stack) {
       // Make sure we are in the right place.
@@ skipping 28 matching lines @@
       evals.add(instance.evaluate('this + frog').then((result) {
             expect(result.type, equals('Error'));
           }));
       return Future.wait(evals);
   });
 },
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: testFunction);