Support Dart core libraries in source maps.

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 f155374d7bf82e536e4d270da6a1223cda9cdfea..7aa5eac1b673b860c10c823dc955670e0f9df3cd 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
@@ -17,6 +17,7 @@ import '../io.dart';
 import '../log.dart' as log;
 import '../package.dart';
 import '../package_graph.dart';
+import '../sdk.dart' as sdk;
 import 'admin_server.dart';
 import 'build_directory.dart';
 import 'dart_forwarding_transformer.dart';
@@ -205,6 +206,16 @@ class BuildEnvironment {
     }).then((_) => url);
   }
 
+  /// Gets the build directory that contains [assetPath] within the entrypoint
+  /// package.
+  ///
+  /// If [assetPath] is not contained within a build directory, this will
+  /// throw an exception.
+  String getBuildDirectoryContaining(String assetPath) =>
+      _directories.values
+          .firstWhere((dir) => path.isWithin(dir.directory, assetPath))
+          .directory;
+
   /// Return all URLs serving [assetPath] in this environment.
   List<Uri> getUrlsForAssetPath(String assetPath) {
     // Check the three (mutually-exclusive) places the path could be pointing.
@@ -339,6 +350,15 @@ class BuildEnvironment {
           path.join('lib', path.relative(library, from: dartPath)));
     });
 
+    // "$sdk" is a pseudo-package that allows the dart2js transformer to find
+    // the Dart core libraries without hitting the file system directly. This
+    // ensures they work with source maps.
+    var libPath = path.join(sdk.rootDirectory, "lib");
+    var sdkSources = listDir(libPath, recursive: true)
+        .where((file) => path.extension(file) == ".dart")
+        .map((file) => new AssetId('\$sdk',
+            path.join("lib", path.relative(file, from: sdk.rootDirectory))));
+
     // Bind a server that we can use to load the transformers.
     var transformerServer;
     return BarbackServer.bind(this, _hostname, 0, null).then((server) {
@@ -346,6 +366,7 @@ class BuildEnvironment {
 
       return log.progress("Loading source assets", () {
         barback.updateSources(pubSources);
+        barback.updateSources(sdkSources);
         return _provideSources();
       });
     }).then((_) {