warn if inlining the same css file more than once

pkg/polymer/lib/src/build/messages.dart

Index: pkg/polymer/lib/src/build/messages.dart
diff --git a/pkg/polymer/lib/src/build/messages.dart b/pkg/polymer/lib/src/build/messages.dart
index 51795db6a0eaa752415c9673692a35fcc83e2437..46ccd4ca92dac027c784843483d4ea737c9c1b9a 100644
--- a/pkg/polymer/lib/src/build/messages.dart
+++ b/pkg/polymer/lib/src/build/messages.dart
@@ -539,3 +539,22 @@ Custom element found in document body without an "unresolved" attribute on it or
 one of its parents. This means your app probably has a flash of unstyled content
 before it finishes loading. See <http://goo.gl/iN03Pj> for more info.
 ''');
+
+const CSS_FILE_INLINED_MULTIPLE_TIMES = const MessageTemplate(
+    const MessageId('polymer', 42),
+    'A css file is being inlined in multiple places for a single entry point.',

[Siggi Cherem (dart-lang), 2014/09/19 21:13:36]

Some ideas:
- 'is being inlined' => 'was inlined'
- 'in multiple places' => 'multiple times'
- 'for a single entrypoint' => 'in your app' (or remove)
- use the template to actually indicate the url of the actual css file

So it could look something like this:

'The css file "%-url-%" was inlined multiple times.'

Since we also have the option of putting a parameter, we could also mention here
what the line fix could look like:

  inline_stylesheets:
    %-canonical-path-%: false

if you like.

[jakemac, 2014/09/19 21:38:16]

Added in the %-url-% placeholder, but we can't put them in the details right?
(thats part of the static site).

[Siggi Cherem (dart-lang), 2014/09/19 22:11:34]

Ah, sorry I meant if you wanted to include some details also in the error
message, for example:

The css file %-url-% was inlined multiple times. You can disable inlining for
this url by adding the following in your pubspec:
    inline_stylesheets:
       %-canonical-path-%: false


Now that I write this down, it feels like the message is getting long and the
formatting is going to be hard, so I'm ok only mentioning the generic
instructions in the site and keeping this short like you have it. 

+    'Css file inlined multiple times',
+    '''
+Css files are inlined by default, but if you import the same one in multiple
+places you probably want to override this behavior to prevent duplicate code.
+To do this, use the following pattern to update your pubspec.yaml:
+
+    transformers:
+    - polymer:
+      inline_stylesheets:
+        web/my_file.css: false
+
+If you would like to hide this warning and keep it inlined, do the same thing
+but assign the value to true.
+'''
+);