Gracefully handle pubspecs with dependencies using unknown sources.

sdk/lib/_internal/pub/test/install/unknown_source_test.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS d.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 d.file.
+
+library pub_tests;
+
+import 'dart:io';
+
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+
+main() {
+  initConfig();
+  integration('fails gracefully on a dependency from an unknown source', () {
+    d.appDir([{"bad": "foo"}]).create();
+
+    schedulePub(args: ['install'],
+        error: new RegExp("Package 'myapp' depends on 'foo' from unknown "
+                          "source 'bad'.\$"),
+        exitCode: 1);
+  });
+
+  integration('fails gracefully on transitive dependency from an unknown '
+              'source', () {
+    d.dir('foo', [
+      d.libDir('foo', 'foo 0.0.1'),
+      d.libPubspec('foo', '0.0.1', deps: [{"bad": "bar"}])
+    ]).create();
+
+    d.appDir([{"path": "../foo"}]).create();
+
+    schedulePub(args: ['install'],
+        error: new RegExp("Package 'foo' depends on 'bar' from unknown "
+                          "source 'bad'.\$"),
+        exitCode: 1);
+  });
+}