Add a "global activate" command to pub.

sdk/lib/_internal/pub/lib/src/lock_file.dart

Index: sdk/lib/_internal/pub/lib/src/lock_file.dart
diff --git a/sdk/lib/_internal/pub/lib/src/lock_file.dart b/sdk/lib/_internal/pub/lib/src/lock_file.dart
index a4cba8f776f7e254a34741bb398ec02ad0219893..95421a604fc372af3f170a0fc6c2f9142b51db7f 100644
--- a/sdk/lib/_internal/pub/lib/src/lock_file.dart
+++ b/sdk/lib/_internal/pub/lib/src/lock_file.dart
@@ -17,6 +17,16 @@ class LockFile {
   /// The packages this lockfile pins.
   Map<String, PackageId> packages;
 
+  /// Creates a new lockfile containing [ids].
+  factory LockFile(List<PackageId> ids) {
+    var lockFile = new LockFile.empty();
+    for (var id in ids) {
+      if (!id.isRoot) lockFile.packages[id.name] = id;
+    }
+
+    return lockFile;
+  }
+
   LockFile._(this.packages);
 
   LockFile.empty()