Implement 'print', 'up', 'down', and 'frame' commands in the Observatory debugger.

runtime/observatory/test/source_location_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.
 
 import 'package:observatory/service_io.dart';
 import 'package:observatory/debugger.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 import 'dart:async';
 
 void testFunction() {
   int i = 0;
   while (true) {
     if (++i % 100000000 == 0) {  // line 14
       print(i);
     }
   }
 }
 
 class TestDebugger extends Debugger {
   TestDebugger(this.isolate, this.stack);
 
   Isolate isolate;
   ServiceMap stack;
+  int currentFrame = 0;
 }
 
 void source_location_dummy_function() {
 }
 
 class SourceLocationTestFoo {
   SourceLocationTestFoo(this.field);
   SourceLocationTestFoo.named();
 
   void method() {}
@@ skipping 234 matching lines @@
     return SourceLocation.complete(debugger, 'SourceLocationTestFoo.q')
       .then((List<String> completions) {
         expect(completions.toString(), equals('[]'));
       });
   });
 },
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: testFunction);