First pass at asynchronous input loading in dart2js.

sdk/lib/_internal/compiler/implementation/source_file_provider.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.
 
 library source_file_provider;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ skipping 20 matching lines @@
   int dartCharactersRead = 0;
 
   Future<String> readStringFromUri(Uri resourceUri) {
     if (resourceUri.scheme != 'file') {
       throw new ArgumentError("Unknown scheme in uri '$resourceUri'");
     }
     String source;
     try {
       source = readAll(uriPathToNative(resourceUri.path));
     } on FileException catch (ex) {
-      throw "Error reading '${relativize(cwd, resourceUri, isWindows)}' "
-            "(${ex.osError})";
+      return new Future.error(
+          "Error reading '${relativize(cwd, resourceUri, isWindows)}' "
+          "(${ex.osError})");
     }
     dartCharactersRead += source.length;
     sourceFiles[resourceUri.toString()] = new SourceFile(
         relativize(cwd, resourceUri, isWindows), source);
     return new Future.value(source);
   }
 
   Future<String> call(Uri resourceUri) => readStringFromUri(resourceUri);
 }
 
@@ skipping 78 matching lines @@
     if (fatal && throwOnError) {
       isAborting = true;
       throw new AbortLeg(message);
     }
   }
 
   void call(Uri uri, int begin, int end, String message, api.Diagnostic kind) {
     return diagnosticHandler(uri, begin, end, message, kind);
   }
 }