Add "allowedExtensions" to Transformer.

pkg/barback/lib/src/transformer.dart

Index: pkg/barback/lib/src/transformer.dart
diff --git a/pkg/barback/lib/src/transformer.dart b/pkg/barback/lib/src/transformer.dart
index 58377f0257fd0c1c51d1bf492e005726d3f4a4c3..9baf3b987fda4ac1935786fe5984f4cd4dd37b55 100644
--- a/pkg/barback/lib/src/transformer.dart
+++ b/pkg/barback/lib/src/transformer.dart
@@ -16,6 +16,15 @@ import 'transform.dart';
 /// files are all examples of transformers. To define your own transformation
 /// step, extend (or implement) this class.
 abstract class Transformer {
+  /// Override this to return a space-separated list of file extensions
+  /// (without leading `.`) that are allowed for the primary inputs to this

[nweiz, 2013/08/20 00:35:43]

It's confusing that this doesn't use the leading ".", but all the AssetId
methods do.

[Bob Nystrom, 2013/08/20 16:15:37]

Changed to require the leading ".".

+  /// transformer.
+  ///
+  /// If you don't override [isPrimary] yourself, it defaults to allowing any
+  /// asset whose extension matches one of the ones returned by this. If you
+  /// don't override [isPrimary] *or* this, it allows all files.
+  String get allowedExtensions => null;

[nweiz, 2013/08/20 00:35:43]

It's weird that this uses a space-separated list when it's just going to be
defined in code. It feels like it should be able to return either a string (for
a single extension) or a list of strings (for multiple).

[Bob Nystrom, 2013/08/20 16:15:37]

I thought about that, but then I wouldn't be able to type the parameter. I could
always require a list, but that punishes the common case of only allowing a
single extension. This was the best compromise I could think of. Since this is
really just a convenience anyway, I wanted to optimize for brevity.

[nweiz, 2013/08/20 19:20:54]

If the way you specify a list of extensions is as a string, you're not really
typing the parameter now. I'd rather have the signature always explicitly
indicate that this can return multiple types of values.

+
   /// Returns `true` if [input] can be a primary input for this transformer.
   ///
   /// While a transformer can read from multiple input files, one must be the
@@ -28,7 +37,20 @@ abstract class Transformer {
   /// of those to generate the final JS. However you still run dart2js "on" a
   /// single file: the entrypoint Dart file that has your `main()` method.
   /// This entrypoint file would be the primary input.
-  Future<bool> isPrimary(Asset input);
+  ///
+  /// If this is not overridden, defaults to allow any asset whose extension
+  /// matches one of the ones returned by [allowedExtensions]. If *that* is
+  /// not overridden, allows all assets.
+  Future<bool> isPrimary(Asset input) {
+    // Allow all files if [primaryExtensions] is not overridden.
+    if (allowedExtensions == null) return new Future.value(true);
+
+    for (var extension in allowedExtensions.split(" ")) {
+      if (input.id.extension == ".$extension") return new Future.value(true);
+    }
+
+    return new Future.value(false);
+  }
 
   /// Run this transformer on on the primary input specified by [transform].
   ///