Handle a null dependencies map in lockfile.

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 1799191bbe28fdc68382506fc30b08904080bed6..e3dff8011d197a68428044a11fd0d96ebefed0d9 100644
--- a/sdk/lib/_internal/pub/lib/src/lock_file.dart
+++ b/sdk/lib/_internal/pub/lib/src/lock_file.dart
@@ -40,9 +40,8 @@ class LockFile {
     if (contents.trim() == '') return new LockFile.empty();
     var parsed = loadYaml(contents);
 
-    if (parsed.containsKey('packages')) {
-      var packageEntries = parsed['packages'];
-
+    var packageEntries = parsed['packages'];
+    if (packageEntries != null) {
       packageEntries.forEach((name, spec) {
         // Parse the version.
         if (!spec.containsKey('version')) {