Support dart2js in pub serve.

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

Index: sdk/lib/_internal/pub/lib/src/dart.dart
diff --git a/sdk/lib/_internal/pub/lib/src/dart.dart b/sdk/lib/_internal/pub/lib/src/dart.dart
index f6d8b11074aa9f71f942e6434db7b0e698f7aa2c..b934112bb38a8e8cef9bcb689a094fac63616f64 100644
--- a/sdk/lib/_internal/pub/lib/src/dart.dart
+++ b/sdk/lib/_internal/pub/lib/src/dart.dart
@@ -25,17 +25,22 @@ import 'utils.dart';
 /// true).
 ///
 /// By default, the package root is assumed to be adjacent to [entrypoint], but
-/// if [packageRoot] is passed that will be used instead.
+/// if [packageRoot] is passed that will be used instead. If [provider] is
+/// omitted, uses a default [SourceFileProvider] that loads directly from the
+/// filesystem.
 Future<String> compile(String entrypoint, {String packageRoot,
-    bool toDart: false}) {
+    bool toDart: false, SourceFileProvider provider}) {
   return new Future.sync(() {
-    var provider = new SourceFileProvider();
     var options = <String>['--categories=Client,Server', '--minify'];
     if (toDart) options.add('--output-type=dart');
     if (packageRoot == null) {
       packageRoot = path.join(path.dirname(entrypoint), 'packages');
     }
 
+    if (provider == null) {
+      provider = new SourceFileProvider();
+    }
+
     return compiler.compile(
         path.toUri(entrypoint),
         path.toUri(appendSlash(_libPath)),
@@ -49,8 +54,10 @@ Future<String> compile(String entrypoint, {String packageRoot,
   });
 }
 
-/// Returns the path to the library directory. This corresponds to the "sdk"
-/// directory in the repo and to the root of the compiled SDK.
+/// Returns the path to the directory containing the Dart core libraries.
+///
+/// This corresponds to the "sdk" directory in the repo and to the root of the
+/// compiled SDK.
 String get _libPath {
   if (runningFromSdk) return sdk.rootDirectory;
   return path.join(repoRoot, 'sdk');