Add more metadata to non-programmatic barback exceptions.

pkg/barback/lib/src/utils.dart

Index: pkg/barback/lib/src/utils.dart
diff --git a/pkg/barback/lib/src/utils.dart b/pkg/barback/lib/src/utils.dart
index 7529fa974d580e91bd4b23531d43b02e7552e40a..bee60841dfdfed85a67ae21a7a3500d81e85f63a 100644
--- a/pkg/barback/lib/src/utils.dart
+++ b/pkg/barback/lib/src/utils.dart
@@ -6,6 +6,23 @@ library barback.utils;
 
 import 'dart:async';
 
+/// A pair of values.
+class Pair<E, F> {
+  E first;
+  F last;
+
+  Pair(this.first, this.last);
+
+  String toString() => '($first, $last)';
+
+  bool operator==(other) {
+    if (other is! Pair) return false;
+    return other.first == first && other.last == last;
+  }
+
+  int get hashCode => first.hashCode ^ last.hashCode;
+}
+
 /// Converts a number in the range [0-255] to a two digit hex string.
 ///
 /// For example, given `255`, returns `ff`.
@@ -46,6 +63,10 @@ List flatten(Iterable nested) {
   return result;
 }
 
+/// Returns the union of all elements in each set in [sets].
+Set unionAll(Iterable<Set> sets) =>
+  sets.fold(new Set(), (union, set) => union.union(set));
+
 /// Passes each key/value pair in [map] to [fn] and returns a new [Map] whose
 /// values are the return values of [fn].
 Map mapMapValues(Map map, fn(key, value)) =>