Don't allow overlapping source directories.

sdk/lib/_internal/pub/lib/src/barback/build_environment.dart

Index: sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart b/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
index 78a459cdd82ca4a7a51639e5b720c2bc783f1491..a332dd5e121a386a99911d1294ddab7ab8b5b510 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
@@ -18,7 +18,6 @@ import '../log.dart' as log;
 import '../package.dart';
 import '../package_graph.dart';
 import '../sdk.dart' as sdk;
-import '../utils.dart';
 import 'admin_server.dart';
 import 'build_directory.dart';
 import 'dart_forwarding_transformer.dart';
@@ -170,6 +169,16 @@ class BuildEnvironment {
       });
     }
 
+    // See if the new directory overlaps any existing servers.
+    var overlapping = _directories.keys.where((directory) =>
+        path.isWithin(directory, rootDirectory) ||
+        path.isWithin(rootDirectory, directory)).toList();
+
+    if (overlapping.isNotEmpty) {
+      return new Future.error(
+          new OverlappingSourceDirectoryException(overlapping));
+    }
+
     var port = _basePort;
 
     // If not using an ephemeral port, find the lowest-numbered available one.
@@ -669,6 +678,16 @@ void _log(LogEntry entry) {
   }
 }
 
+/// Exception thrown when trying to serve a new directory that overlaps one or
+/// more directories already being served.
+class OverlappingSourceDirectoryException implements Exception {
+  /// The relative paths of the directories that overlap the one that could not
+  /// be served.
+  final List<String> overlappingDirectories;
+
+  OverlappingSourceDirectoryException(this.overlappingDirectories);
+}
+
 /// An enum describing different modes of constructing a [DirectoryWatcher].
 abstract class WatcherType {
   /// A watcher that automatically chooses its type based on the operating