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

runtime/observatory/lib/src/debugger/source_location.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.
 
 part of debugger;
 
 class SourceLocation {
   SourceLocation.file(this.script, this.line, this.col);
   SourceLocation.func(this.function);
   SourceLocation.error(this.errorMessage);
@@ skipping 30 matching lines @@
     return new Future.value(new SourceLocation.error(
         "Invalid source location '${locDesc}'"));
   }
 
   static Future<SourceLocation> _currentLocation(Debugger debugger) {
     ServiceMap stack = debugger.stack;
     if (stack == null || stack['frames'].length == 0) {
       return new Future.value(new SourceLocation.error(
           'A script must be provided when the stack is empty'));
     }
-    var frame = stack['frames'][0];
+    var frame = stack['frames'][debugger.currentFrame];
     Script script = frame['script'];
     return script.load().then((_) {
       var line = script.tokenToLine(frame['tokenPos']);
       // TODO(turnidge): Pass in the column here once the protocol supports it.
       return new Future.value(new SourceLocation.file(script, line, null));
     });
   }
 
   static Future<SourceLocation> _parseScriptLine(Debugger debugger,
                                                  Match match) {
@@ skipping 296 matching lines @@
     return 'invalid source location (${errorMessage})';
   }
 
   Script script;
   int line;
   int col;
   ServiceFunction function;
   String errorMessage;
   bool get valid => (errorMessage == null);
 }