Don't run "git fetch" multiple times for the same repository.

sdk/lib/_internal/pub/lib/src/source/git.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.source.git;
 
 import 'dart:async';
 
 import 'package:path/path.dart' as path;
 
 import '../git.dart' as git;
 import '../io.dart';
 import '../package.dart';
 import '../source.dart';
 import '../utils.dart';
 
 /// A package source that gets packages from Git repos.
 class GitSource extends Source {
   final String name = "git";
 
   final bool shouldCache = true;
 
+  /// The paths to the canonical clones of repositories for which "git fetch"
+  /// has already been run during this run of pub.
+  final _updatedRepos = new Set<String>();
+
   GitSource();
 
   /// Clones a Git repo to the local filesystem.
   ///
   /// The Git cache directory is a little idiosyncratic. At the top level, it
   /// contains a directory for each commit of each repository, named `<package
   /// name>-<commit hash>`. These are the canonical package directories that are
   /// linked to from the `packages/` directory.
   ///
   /// In addition, the Git system cache contains a subdirectory named `cache/`
@@ skipping 111 matching lines @@
         if (error is! GitException) throw error;
         return _updateRepoCache(id).then((_) => _revParse(id));
       });
     });
   }
 
   /// Runs "git fetch" in the canonical clone of the repository referred to by
   /// [id].
   ///
   /// This assumes that the canonical clone already exists.
-  Future _updateRepoCache(PackageId id) =>
-      git.run(["fetch"], workingDir: _repoCachePath(id));
+  Future _updateRepoCache(PackageId id) {
+    var path = _repoCachePath(id);
+    if (_updatedRepos.contains(path)) return new Future.value();
+    return git.run(["fetch"], workingDir: path).then((_) {
+      _updatedRepos.add(path);
+    });
+  }
 
   /// Runs "git rev-parse" in the canonical clone of the repository referred to
   /// by [id] on the effective ref of [id].
   ///
   /// This assumes that the canonical clone already exists.
   Future<String> _revParse(PackageId id) {
     return git.run(["rev-parse", _getEffectiveRef(id)],
         workingDir: _repoCachePath(id)).then((result) => result.first);
   }
 
@@ skipping 63 matching lines @@
     return description['ref'];
   }
 
   /// Returns [description] if it's a description, or [PackageId.description] if
   /// it's a [PackageId].
   _getDescription(description) {
     if (description is PackageId) return description.description;
     return description;
   }
 }