Migrate pub away from throwing strings.

utils/pub/entrypoint.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 entrypoint;
 
 import 'dart:async';
 
 import 'package:pathos/path.dart' as path;
 
@@ skipping 69 matching lines @@
         log.fine("Deleting package directory for ${id.name} before install.");
         deleteEntry(packageDir);
       }
 
       if (id.source.shouldCache) {
         return cache.install(id).then(
             (pkg) => createPackageSymlink(id.name, pkg.dir, packageDir));
       } else {
         return id.source.install(id, packageDir).then((found) {
           if (found) return null;
-          // TODO(nweiz): More robust error-handling.
-          throw 'Package ${id.name} not found in source "${id.source.name}".';
+          fail('Package ${id.name} not found in source "${id.source.name}".');
         });
       }
     }).then((_) => id.resolved);
 
     _installs[id] = future;
 
     return future;
   }
 
   /// Installs all dependencies of the [root] package to its "packages"
@@ skipping 174 matching lines @@
   }
 
   /// Creates a symlink to the `packages` directory in [dir]. Will replace one
   /// if already there.
   void _linkSecondaryPackageDir(String dir) {
     var symlink = path.join(dir, 'packages');
     if (entryExists(symlink)) deleteEntry(symlink);
     createSymlink(packagesDir, symlink, relative: true);
   }
 }