Validate packages against their SDK constraints.

utils/pub/utils.dart

Index: utils/pub/utils.dart
diff --git a/utils/pub/utils.dart b/utils/pub/utils.dart
index 7710ac194a1691e2da88c94fad9f780979d93f07..616397578b8a2650979ba13dbec52be48b9a7278 100644
--- a/utils/pub/utils.dart
+++ b/utils/pub/utils.dart
@@ -27,6 +27,47 @@ class Pair<E, F> {
   int get hashCode => first.hashCode ^ last.hashCode;
 }
 
+/// A completer that waits until all added [Future]s complete.
+// TODO(rnystrom): Copied from web_components. Remove from here when it gets
+// added to dart:core. (See #6626.)
+class FutureGroup<T> {
+  int _pending = 0;
+  Completer<List<T>> _completer = new Completer<List<T>>();
+  final List<Future<T>> futures = <Future<T>>[];
+  bool completed = false;
+
+  final List<T> _values = <T>[];
+
+  /// Wait for [task] to complete.
+  Future<T> add(Future<T> task) {
+    if (completed) {
+      throw new StateError("The FutureGroup has already completed.");
+    }
+
+    _pending++;
+    futures.add(task.then((value) {
+      if (completed) return;
+
+      _pending--;
+      _values.add(value);
+
+      if (_pending <= 0) {
+        completed = true;
+        _completer.complete(_values);
+      }
+    }).catchError((e) {
+      if (completed) return;
+
+      completed = true;
+      _completer.completeError(e.error, e.stackTrace);
+    }));
+
+    return task;
+  }
+
+  Future<List> get future => _completer.future;
+}
+
 // TODO(rnystrom): Move into String?
 /// Pads [source] to [length] by adding spaces at the end.
 String padRight(String source, int length) {