Insert debug step check at the beginning of a function.

runtime/observatory/tests/service/break_on_function_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  --verbose_debug
+
+import 'package:observatory/service_io.dart';
+import 'package:unittest/unittest.dart';
+import 'test_helper.dart';
+import 'dart:developer';
+
+testFunction(flag) {  // Line 11
+  if (flag) {
+    print("Yes");
+  } else {
+    print("No");
+  }
+}
+
+testMain() {
+  debugger();
+  testFunction(true);
+  testFunction(false);
+  print("Done");
+}
+
+var tests = [
+
+hasStoppedAtBreakpoint,
+
+// Add breakpoint
+(Isolate isolate) async {
+  var rootLib = await isolate.rootLibrary.load();
+  var function = rootLib.functions.singleWhere((f) => f.name == 'testFunction');
+
+  var bpt = await isolate.addBreakpointAtEntry(function);
+  expect(bpt is Breakpoint, isTrue);
+  print(bpt);
+},
+
+resumeIsolate,
+
+hasStoppedAtBreakpoint,
+
+// We are at the breakpoint on line 11.
+(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(stack['frames'][0].location.tokenPos, equals(22));
+},
+
+resumeIsolate,
+
+hasStoppedAtBreakpoint,
+
+// We are at the breakpoint on line 11.
+(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(stack['frames'][0].location.tokenPos, equals(22));
+},
+
+resumeIsolate,
+
+];
+
+main(args) => runIsolateTests(args, tests, testeeConcurrent: testMain);