Use precompiled dependencies in 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 1b22647b694f8600f650604f36aec63938b33086..e0b218590973d965b33b402581c523e06825a905 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
@@ -125,8 +125,7 @@ class Dart2JSTransformer extends Transformer implements LazyTransformer {
     // against.
     var id = transform.primaryInput.id;
 
-    var entrypoint = path.join(_environment.graph.packages[id.package].dir,
-        id.path);
+    var entrypoint = _environment.graph.packages[id.package].path(id.path);
 
     // TODO(rnystrom): Should have more sophisticated error-handling here. Need
     // to report compile errors to the user in an easily visible way. Need to
@@ -141,7 +140,7 @@ class Dart2JSTransformer extends Transformer implements LazyTransformer {
             'minify', defaultsTo: _settings.mode == BarbackMode.RELEASE),
         verbose: _configBool('verbose'),
         environment: _configEnvironment,
-        packageRoot: path.join(_environment.rootPackage.dir, "packages"),
+        packageRoot: _environment.rootPackage.path("packages"),
         analyzeAll: _configBool('analyzeAll'),
         suppressWarnings: _configBool('suppressWarnings'),
         suppressHints: _configBool('suppressHints'),
@@ -260,7 +259,7 @@ class _BarbackCompilerProvider implements dart.CompilerProvider {
     // TODO(rnystrom): Fix this if #17751 is fixed.
     var buildDir = _environment.getSourceDirectoryContaining(
         _transform.primaryInput.id.path);
-    _libraryRootPath = path.join(_environment.rootPackage.dir,
+    _libraryRootPath = _environment.rootPackage.path(
         buildDir, "packages", r"$sdk");
   }
 
@@ -395,8 +394,8 @@ class _BarbackCompilerProvider implements dart.CompilerProvider {
     // should be loaded directly from disk.
     var sourcePath = path.fromUri(url);
     if (_environment.containsPath(sourcePath)) {
-      var relative = path.toUri(path.relative(sourcePath,
-          from: _environment.rootPackage.dir)).toString();
+      var relative = path.toUri(_environment.rootPackage.relative(sourcePath))
+          .toString();
 
       return new AssetId(_environment.rootPackage.name, relative);
     }