Add stack chain support to pub.

sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart

Index: sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
index c4b0597938e2d33cdc1729118e828aa5fd8cace6..f21ed9e4a638b17ee3f3e6c0ee5dca64ff0f239c 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
@@ -11,6 +11,7 @@ import 'dart:io';
 import 'package:analyzer/analyzer.dart';
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
+import 'package:stack_trace/stack_trace.dart';
 
 import '../../../../compiler/compiler.dart' as compiler;
 import '../../../../compiler/implementation/dart2js.dart'
@@ -101,9 +102,10 @@ class Dart2JSTransformer extends Transformer {
       // Need to report compile errors to the user in an easily visible way.
       // Need to make sure paths in errors are mapped to the original source
       // path so they can understand them.
-      return dart.compile(entrypoint, provider,
+      return Chain.track(dart.compile(
+          entrypoint, provider,
           packageRoot: packageRoot,
-          minify: _mode == BarbackMode.RELEASE).then((_) {
+          minify: _mode == BarbackMode.RELEASE)).then((_) {
         stopwatch.stop();
         transform.logger.info("Took ${stopwatch.elapsed} to compile $id.");
       });
@@ -266,7 +268,7 @@ class _BarbackCompilerProvider implements dart.CompilerProvider {
     // skip Barback and just hit the file system. This will occur at the very
     // least for dart2js's implementations of the core libraries.
     var sourcePath = path.fromUri(url);
-    return new File(sourcePath).readAsString();
+    return Chain.track(new File(sourcePath).readAsString());
   }
 
   AssetId _sourceUrlToId(Uri url) {