Implicitly constrain barback to versions pub supports.

pkg/barback/pubspec.yaml

Index: pkg/barback/pubspec.yaml
diff --git a/pkg/barback/pubspec.yaml b/pkg/barback/pubspec.yaml
index 54e3a1ac25af54ac3e6e008fca95926f029c2336..3032f3d2e20f6d71e554f6f29f53980fd5f3d3b2 100644
--- a/pkg/barback/pubspec.yaml
+++ b/pkg/barback/pubspec.yaml
@@ -1,5 +1,15 @@
 name: barback
-version: 0.9.0 # Replaced by publish_barback.py. Do not edit.
+
+# Note! This version is referenced directly in the pub source code in
+# lib/src/barback.dart. Pub implicitly places a version constraint on barback
+# to ensure users only select a version of barback that works with their current
+# version of pub.
+#
+# When the minor version of this is upgraded, you *must* update that version
+# number in pub to stay in sync with this. New patch versions are considered
+# backwards compatible, and pub will allow later patch versions automatically.
+version: 0.10.2+1
+
 author: "Dart Team <misc@dartlang.org>"
 homepage: http://www.dartlang.org
 description: >
@@ -19,14 +29,5 @@ dependencies:
 dev_dependencies:
   scheduled_test: ">=0.9.0 <0.10.0"
   unittest: ">=0.9.0 <0.10.0"
-
 environment:
-  # Barback is tightly coupled to the SDK because pub contains code that is run
-  # against the user's version of barback. We need to ensure that that version
-  # of barback is compatible with the user's version of pub.
-  #
-  # Since the SDK itself can't place constraints on barback, we do a reverse
-  # constraint and have barback itself only allow specific SDK versions.
-  # This ensures that for each SDK version, there is a single known good version
-  # of barback that will be used with it.
-  sdk: "$SDK_CONSTRAINT$"
+  sdk: ">=1.0.1 <2.0.0"