Support dart2js in pub serve.

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

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// A library for compiling Dart code and manipulating analyzer parse trees.
 library pub.dart;
 
 import 'dart:async';
 import 'dart:isolate';
 
 import 'package:analyzer_experimental/analyzer.dart';
 import 'package:path/path.dart' as path;
 import 'package:stack_trace/stack_trace.dart';
 import '../../../compiler/compiler.dart' as compiler;
 import '../../../compiler/implementation/source_file_provider.dart'
     show FormattingDiagnosticHandler, SourceFileProvider;
 import '../../../compiler/implementation/filenames.dart'
     show appendSlash;
 
 import 'io.dart';
 import 'sdk.dart' as sdk;
 import 'utils.dart';
 
 /// Returns [entrypoint] compiled to JavaScript (or to Dart if [toDart] is
 /// 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)),
         path.toUri(appendSlash(packageRoot)),
         provider.readStringFromUri,
         new FormattingDiagnosticHandler(provider).diagnosticHandler,
         options);
   }).then((result) {
     if (result != null) return result;
     throw new ApplicationException('Failed to compile "$entrypoint".');
   });
 }
 
-/// 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');
 }
 
 /// Returns whether [dart] looks like an entrypoint file.
 bool isEntrypoint(CompilationUnit dart) {
   // TODO(nweiz): this misses the case where a Dart file doesn't contain main(),
   // but it parts in another file that does.
   return dart.declarations.any((node) {
@@ skipping 82 matching lines @@
     if (stack == null) stack = getAttachedStackTrace(error);
     return {
       'type': error.runtimeType.toString(),
       'message': getErrorMessage(error),
       'stack': stack == null ? null : stack.toString()
     };
   }
 
   String toString() => "$message\n$stackTrace";
 }