Add implicit constraints from pub onto stack_trace and source_maps.

These packages are used in pub's plugin isolate, so pub needs to have
a dependency on them to ensure their APIs don't change out from under
it.

R=rnystrom@google.com
BUG=19574

Review URL: https://codereview.chromium.org//356523003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_maps@37751 260f80e4-7a28-3924-810f-c04153c831b5

pkgs/source_maps/pubspec.yaml

diff --git a/pkgs/source_maps/pubspec.yaml b/pkgs/source_maps/pubspec.yaml
index 80c7e7f..97d6c6b 100644
--- a/pkgs/source_maps/pubspec.yaml
+++ b/pkgs/source_maps/pubspec.yaml
@@ -1,4 +1,12 @@
 name: source_maps
+
+# Note! This version number is referenced directly in the pub source code in
+# lib/src/barback.dart. Pub implicitly places a version constraint on
+# source_maps to ensure users only select a version of source_maps that works
+# with their current version of pub.
+#
+# When the minor version is upgraded, you *must* update that version constraint
+# in pub to stay in sync with this.
 version: 0.9.2+2
 author: Dart Team <misc@dartlang.org>
 description: Library to programmatically manipulate source map files.