Gracefully handle pubspecs with dependencies using unknown sources.

sdk/lib/_internal/pub/lib/src/hosted_source.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 hosted_source;
 
 import 'dart:async';
 import 'dart:io' as io;
 import 'dart:json' as json;
 import 'dart:uri';
@@ skipping 30 matching lines @@
           .map((version) => new Version.parse(version))
           .toList();
     }).catchError((ex) {
       var parsed = _parseDescription(description);
       _throwFriendlyError(ex, parsed.first, parsed.last);
     });
   }
 
   /// Downloads and parses the pubspec for a specific version of a package that
   /// is available from the site.
-  Future<Pubspec> describe(PackageId id) {
+  Future<Pubspec> describeUncached(PackageId id) {
     // Request it from the server.
     var url = _makeVersionUrl(id, (server, package, version) =>
         "$server/packages/$package/versions/$version.yaml");
 
     log.io("Describe package at $url.");
     return httpClient.read(url).then((yaml) {
       // TODO(rnystrom): After this is pulled down, we could place it in
       // a secondary cache of just pubspecs. This would let us have a
       // persistent cache for pubspecs for packages that haven't actually
       // been installed.
@@ skipping 122 matching lines @@
       return versions;
     });
   }
 
   Future<bool> install(PackageId id, String destPath) {
     // Since HostedSource returns `true` for [shouldCache], install will only
     // be called for uncached packages.
     throw new UnsupportedError("Cannot install packages when offline.");
   }
 
-  Future<Pubspec> describe(PackageId id) {
+  Future<Pubspec> describeUncached(PackageId id) {
     // [getVersions()] will only return packages that are already cached.
-    // SystemCache should only call [describe()] on a package after it has
+    // Source should only call [describeUncached()] on a package after it has
     // failed to find it in the cache, so this code should not be reached.
     throw new UnsupportedError("Cannot describe packages when offline.");
   }
 }
 
 /// The URL of the default package repository.
 final _defaultUrl = "https://pub.dartlang.org";
 
 String _getSourceDirectory(String url) {
   url = url.replaceAll(new RegExp(r"^https?://"), "");
@@ skipping 43 matching lines @@
   }
 
   var name = description["name"];
   if (name is! String) {
     throw new FormatException("The 'name' key must have a string value.");
   }
 
   var url = description.containsKey("url") ? description["url"] : _defaultUrl;
   return new Pair<String, String>(name, url);
 }