Use a lockfile to persist Pub's installed version constellation.

utils/pub/git_source.dart

Index: utils/pub/git_source.dart
diff --git a/utils/pub/git_source.dart b/utils/pub/git_source.dart
index 7c5fb8c5c994c8301c6f2214b83f56ea5479606a..4efb21bff2889670e2e0e2afbd6657f6ad560f1b 100644
--- a/utils/pub/git_source.dart
+++ b/utils/pub/git_source.dart
@@ -54,8 +54,8 @@ class GitSource extends Source {
       if (exists) return new Future.immediate(null);
       return _clone(_repoCachePath(id), revisionCachePath);
     }).chain((_) {
-      var ref = _getRef(id);
-      if (ref == null) return new Future.immediate(null);
+      var ref = _getEffectiveRef(id);
+      if (ref == 'HEAD') return new Future.immediate(null);
       return _checkOut(revisionCachePath, ref);
     }).chain((_) => Package.load(revisionCachePath, systemCache.sources));
   }
@@ -70,7 +70,7 @@ class GitSource extends Source {
   /**
    * Ensures [description] is a Git URL.
    */
-  void validateDescription(description) {
+  void validateDescription(description, [bool fromLockFile = false]) {
     // A single string is assumed to be a Git URL.
     if (description is String) return;
     if (description is! Map || !description.containsKey('url')) {
@@ -80,10 +80,11 @@ class GitSource extends Source {
     description = new Map.from(description);
     description.remove('url');
     description.remove('ref');
+    if (fromLockFile) description.remove('resolved-ref');
 
     if (!description.isEmpty()) {
       var plural = description.length > 1;
-      var keys = Strings.join(description.keys, ', ');
+      var keys = Strings.join(description.getKeys(), ', ');
       throw new FormatException("Invalid key${plural ? 's' : ''}: $keys.");
     }
   }
@@ -100,6 +101,17 @@ class GitSource extends Source {
   }
 
   /**
+   * Attaches a specific commit to [id] to disambiguate it.
+   */
+  Future<PackageId> resolveId(PackageId id) {
+    return _revisionAt(id).transform((revision) {
+      var description = {'url': _getUrl(id), 'ref': _getRef(id)};
+      description['resolved-ref'] = revision;
+      return new PackageId(this, id.version, description);
+    });
+  }
+
+  /**
    * Ensure that the canonical clone of the repository referred to by [id] (the
    * one in `<system cache>/git/cache`) exists and is up-to-date. Returns a
    * future that completes once this is finished and throws an exception if it
@@ -119,11 +131,10 @@ class GitSource extends Source {
   }
 
   /**
-   * Returns a future that completes to the revision hash of the repository for
-   * [id] at [ref], which can be any Git ref.
+   * Returns a future that completes to the revision hash of [id].
    */
-  Future<String> _revisionAt(PackageId id, String ref) {
-    return runProcess("git", ["rev-parse", ref],
+  Future<String> _revisionAt(PackageId id) {
+    return runProcess("git", ["rev-parse", _getEffectiveRef(id)],
         workingDir: _repoCachePath(id), pipeStderr: true).transform((result) {
       if (!result.success) throw 'Git failed.';
       return result.stdout[0];
@@ -131,13 +142,10 @@ class GitSource extends Source {
   }
 
   /**
-   * Returns the path to the revision-specific cache of [id] at [ref], which can
-   * be any Git ref.
+   * Returns the path to the revision-specific cache of [id].
    */
   Future<String> _revisionCachePath(PackageId id) {
-    var ref = _getRef(id);
-    if (ref == null) ref = 'HEAD';
-    return _revisionAt(id, ref).transform((rev) {
+    return _revisionAt(id).transform((rev) {
       var revisionCacheName = '${id.name}-$rev';
       return join(systemCacheRoot, revisionCacheName);
     });
@@ -186,7 +194,26 @@ class GitSource extends Source {
   }
 
   /**
-   * Returns the commit ref for [id], or null if none is given.
+   * Returns the commit ref that should be checked out for [description].
+   *
+   * This differs from [_getRef] in that it doesn't just return the ref in
+   * [description]. It will return a sensible default if that ref doesn't exist,
+   * and it will respect the "resolved-ref" parameter set by [resolveId].
+   *
+   * [description] may be a description or a [PackageId].
+   */
+  String _getEffectiveRef(description) {
+    description = _getDescription(description);
+    if (description is Map && description.containsKey('resolved-ref')) {
+      return description['resolved-ref'];
+    }
+
+    var ref = _getRef(description);
+    return ref == null ? 'HEAD' : ref;
+  }
+
+  /**
+   * Returns the commit ref for [description], or null if none is given.
    *
    * [description] may be a description or a [PackageId].
    */