Don't return negative token positions in stack traces.

runtime/observatory/tests/service/positive_token_pos_test.dart

+// Copyright (c) 2016, 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=--error_on_bad_type --error_on_bad_override  --verbose_debug
+
+import 'dart:developer';
+import 'package:observatory/service_io.dart';
+import 'package:unittest/unittest.dart';
+import 'service_test_common.dart';
+import 'test_helper.dart';
+
+helper() async {  // Line 12, Col 16 is the open brace.
+}
+
+testMain() {
+  debugger();
+  helper();  // Line 17, Col 3 is the position of the function call.
+}
+
+var tests = [
+  hasStoppedAtBreakpoint,
+  stoppedAtLine(17),
+  stepInto,
+
+  (Isolate isolate) async {
+    ServiceMap stack = await isolate.getStack();
+    expect(stack['frames'].length, greaterThan(3));
+
+    var frame = stack['frames'][0];
+    expect(frame.function.name, equals('Completer.sync'));
+    expect(await frame.location.getLine(), greaterThan(0));
+    expect(await frame.location.getColumn(), greaterThan(0));
+
+    // We used to return a negative token position for this frame.
+    // See issue #27128.
+    frame = stack['frames'][1];
+    expect(frame.function.name, equals('helper'));
+    expect(await frame.location.getLine(), equals(12));
+    expect(await frame.location.getColumn(), equals(16));
+
+    frame = stack['frames'][2];
+    expect(frame.function.name, equals('testMain'));
+    expect(await frame.location.getLine(), equals(17));
+    expect(await frame.location.getColumn(), equals(3));
+  }
+];
+
+main(args) => runIsolateTests(args, tests, testeeConcurrent: testMain);