Re-land r29957 (add versions and constraints for packages and samples), with

pkg/polymer/pubspec.yaml

 name: polymer
+version: 0.9.0
 author: Polymer.dart Authors <web-ui-dev@dartlang.org>
 description: >
   Polymer.dart is a new type of library for the web, built on top of Web
   Components, and designed to leverage the evolving web platform on modern
   browsers.
 homepage: https://www.dartlang.org/polymer-dart/
 dependencies:
-  analyzer: any
-  args: any
-  barback: any
-  browser: any
-  csslib: any
-  custom_element: any
-  html5lib: any
-  html_import: any
-  logging: any
-  meta: any
-  observe: any
-  path: any
-  polymer_expressions: any
-  shadow_dom: any
-  source_maps: any
-  template_binding: any
-  yaml: any
+  analyzer: ">=0.10.1 <0.11.0"
+  args: ">=0.9.0 <0.10.0"
+  barback: ">=0.9.0 <0.10.0"
+  browser: ">=0.9.0 <0.10.0"
+  csslib: ">=0.9.0 <0.10.0"
+  custom_element: ">=0.9.0 <0.10.0"
+  html5lib: ">=0.9.0 <0.10.0"
+  html_import: ">=0.9.0 <0.10.0"
+  logging: ">=0.9.0 <0.10.0"
+  observe: ">=0.9.0 <0.10.0"
+  path: ">=0.9.0 <0.10.0"
+  polymer_expressions: ">=0.9.0 <0.10.0"
+  shadow_dom: ">=0.9.0 <0.10.0"
+  source_maps: ">=0.9.0 <0.10.0"
+  template_binding: ">=0.9.0 <0.10.0"
+  yaml: ">=0.9.0 <0.10.0"
 dev_dependencies:
-  unittest: any
+  unittest: ">=0.9.0 <0.10.0"
+environment:
+  sdk: ">=0.8.10+6 <2.0.0"