Don't run 'pub list-package-dirs' if there is no pubpsec.lock file.

pkg/analyzer/lib/source/pub_package_map_provider.dart

Index: pkg/analyzer/lib/source/pub_package_map_provider.dart
diff --git a/pkg/analyzer/lib/source/pub_package_map_provider.dart b/pkg/analyzer/lib/source/pub_package_map_provider.dart
index 07fdba7264defe96391118995ccf5c915e8ce6b4..5daf61d6ed8e58634bdd44cf1880d691f7c47ef6 100644
--- a/pkg/analyzer/lib/source/pub_package_map_provider.dart
+++ b/pkg/analyzer/lib/source/pub_package_map_provider.dart
@@ -59,6 +59,13 @@ class PubPackageMapProvider implements PackageMapProvider {
 
   @override
   PackageMapInfo computePackageMap(Folder folder) {
+    // If the pubspec.lock file does not exist, no need to run anything.
+    {
+      String lockPath = getPubspecLockPath(folder);
+      if (!resourceProvider.getFile(lockPath).exists) {
+        return computePackageMapError(folder);
+      }
+    }
     // TODO(paulberry) make this asynchronous so that we can (a) do other
     // analysis while it's in progress, and (b) time out if it takes too long
     // to respond.
@@ -96,13 +103,18 @@ class PubPackageMapProvider implements PackageMapProvider {
     // we'll know when to try running "pub list-package-dirs" again.
     // Unfortunately, "pub list-package-dirs" doesn't tell us dependencies when
     // an error occurs, so just assume there is one dependency, "pubspec.lock".
-    List<String> dependencies = <String>[
-      resourceProvider.pathContext.join(folder.path, PUBSPEC_LOCK_NAME)
-    ];
+    String lockPath = getPubspecLockPath(folder);
+    List<String> dependencies = <String>[lockPath];
     return new PackageMapInfo(null, dependencies.toSet());
   }
 
   /**
+   * Return the path to the `pubspec.lock` file in the given [folder].
+   */
+  String getPubspecLockPath(Folder folder) =>
+      resourceProvider.pathContext.join(folder.path, PUBSPEC_LOCK_NAME);
+
+  /**
    * Decode the JSON output from pub into a package map.  Paths in the
    * output are considered relative to [folder].
    */