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

runtime/observatory/lib/src/debugger/source_location.dart

Index: runtime/observatory/lib/src/debugger/source_location.dart
diff --git a/runtime/observatory/lib/src/debugger/source_location.dart b/runtime/observatory/lib/src/debugger/source_location.dart
index 07ab3f6c00e3903c730a46e2351028b065138cf7..f14d87b14b22c7bb1e2cf4691b7937a558bf4234 100644
--- a/runtime/observatory/lib/src/debugger/source_location.dart
+++ b/runtime/observatory/lib/src/debugger/source_location.dart
@@ -48,7 +48,7 @@ class SourceLocation {
       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']);