First pass at "pub update" implementation.

utils/pub/pub.dart

Index: utils/pub/pub.dart
diff --git a/utils/pub/pub.dart b/utils/pub/pub.dart
index 6ed5b269d50a13a135131cb64e7cf0f7c04cc8df..dae4dea554618f2191dd0b5c114c246676306ea3 100644
--- a/utils/pub/pub.dart
+++ b/utils/pub/pub.dart
@@ -16,6 +16,7 @@
 #source('package.dart');
 
 Map<String, PubCommand> commands;
+PackageCache cache;
 
 main() {
   final args = new Options().arguments;
@@ -25,6 +26,7 @@ main() {
   // be consistent with other Unix apps.
   commands = {
     'list': new PubCommand('print the contents of repositories', commandList),
+    'update': new PubCommand("update a package's dependencies", commandUpdate),
     'version': new PubCommand('print Pub version', commandVersion)
   };
 
@@ -45,6 +47,9 @@ main() {
     }
   }
 
+  // TODO(rnystrom): Do we want this to be global?
+  cache = new PackageCache(cacheDir);
+
   var options = new PubOptions(cacheDir);
 
   // Select the command.
@@ -96,6 +101,17 @@ void commandVersion(PubOptions options, List<String> args) {
   print('Pub 0.0.0');
 }
 
+/**
+ * Gets the package that contains the current working directory. In other words,
+ * finds the package that the user is currently "in".
+ */
+Future<Package> getWorkingPackage() {
+  // TODO(rnystrom): Will eventually need better logic to walk up
+  // subdirectories until we hit one that looks package-like. For now, just
+  // assume the cwd is it.
+  return Package.load(workingDir);
+}
+
 typedef void CommandFunction(PubOptions options, List<String> args);
 
 class PubCommand {