Start sketching out a buildAll() method.

pkg/barback/lib/src/errors.dart

Index: pkg/barback/lib/src/errors.dart
diff --git a/pkg/barback/lib/src/errors.dart b/pkg/barback/lib/src/errors.dart
index 138984ce58200de5d450f9a1b108efcb68f60b64..13886978505f46b51384d6f9853ea2a5600bdb23 100644
--- a/pkg/barback/lib/src/errors.dart
+++ b/pkg/barback/lib/src/errors.dart
@@ -20,11 +20,32 @@ class AssetNotFoundException implements Exception {
   String toString() => "Could not find asset $id.";
 }
 
+/// Recursively removes any occurrences of [AggregateException] in [errors]

[nweiz, 2013/08/20 19:59:26]

"removes" -> "replaces"

Doing this recursively seems unnecessary, since AggregateException is guaranteed
not to contain any other AggregateExceptions.

[Bob Nystrom, 2013/08/20 21:29:20]

Done.

+/// with the list of errors it contains.
+Iterable<BarbackException> flattenAggregateExceptions(
+    Iterable<BarbackException> errors) {
+  return errors.expand((error) {
+    if (error is! AggregateException) return [error];
+    return flattenAggregateExceptions(error.errors);
+  });
+}
+
 /// The interface for exceptions from the barback graph or its transformers.
 ///
 /// These exceptions are never produced by programming errors in barback.
 abstract class BarbackException implements Exception {}
 
+/// An error that wraps a collection of other [BarbackException]s.
+///
+/// It implicitly flattens any [AggregateException]s that occur in the list

[nweiz, 2013/08/20 19:59:26]

"list exceptions" -> "list of exceptions"

[Bob Nystrom, 2013/08/20 21:29:20]

Done.

+/// exceptions it wraps.
+class AggregateException implements BarbackException {
+  final List<BarbackException> errors;

[nweiz, 2013/08/20 19:59:26]

It seems like this should be a set, since the order exceptions come in is going
to be nondeterministic most of the time.

[Bob Nystrom, 2013/08/20 21:29:20]

Done.

+
+  AggregateException(Iterable<BarbackException> errors)
+      : errors = flattenAggregateExceptions(errors).toList();
+}

[nweiz, 2013/08/20 19:59:26]

It would be nice to have a static method here that takes an
Iterable<BarbackException> and does:

    if (errors.length == 0) return null;
    if (errors.length == 1) return errors.single;
    return new AggregateException(errors);

Also, this should have a useful toString.

[Bob Nystrom, 2013/08/20 21:29:20]

Done.

+
 /// Error thrown when two or more transformers both output an asset with [id].
 class AssetCollisionException implements BarbackException {
   /// All the transforms that output an asset with [id].