Lazily throw pubspec parsing exceptions.

sdk/lib/_internal/pub/lib/src/solver/version_solver.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library pub.solver.version_solver;
 
 import 'dart:async';
 import "dart:convert";
 
 import '../lock_file.dart';
@@ skipping 257 matching lines @@
   String _describeDependency(PackageDep dep) =>
       "depends on it from source ${dep.source}";
 }
 
 /// Exception thrown when a dependency on an unknown source name is found.
 class UnknownSourceException extends SolveFailure {
   UnknownSourceException(String package, Iterable<Dependency> dependencies)
       : super(package, dependencies);
 
   String toString() {
-    var dep = only(dependencies);
+    var dep = dependencies.single;
     return "Package '${dep.depender}' depends on '${dep.dep.name}' from "
            "unknown source '${dep.dep.source}'.";
   }
 }
 
 /// Exception thrown when two packages with the same name and source but
 /// different descriptions are depended upon.
 class DescriptionMismatchException extends SolveFailure {
   DescriptionMismatchException(String package,
       Iterable<Dependency> dependencies)
       : super(package, dependencies);
 
   String get _message => "Incompatible dependencies on '$package'";
 
   String _describeDependency(PackageDep dep) {
     // TODO(nweiz): Dump descriptions to YAML when that's supported.
     return "depends on it with description ${JSON.encode(dep.description)}";
   }
 }