Add support for source maps in deferred libraries.

tests/compiler/dart2js/source_map_deferred_d2js_validity_test.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/implementation/dart2js.dart' as entry;
 import 'package:compiler/implementation/apiimpl.dart';
 
 import 'source_map_validator_helper.dart';
 import 'compiler_alt.dart' as alt;
 
 void main() {
   entry.compileFunc = alt.compile;
 
   asyncTest(() => createTempDir().then((Directory tmpDir) {
-    print(
-        'Compiling tests/compiler/dart2js/source_map_validator_test_file.dart');
+    String file =
+        'tests/compiler/dart2js/source_map_deferred_validator_test_file.dart';
+    print("Compiling $file");
     Future result = entry.internalMain(
-        ['tests/compiler/dart2js/source_map_validator_test_file.dart',
+        [file,
          '-o${tmpDir.path}/out.js',
          '--library-root=sdk']);
       return result.then((_) {
         Compiler compiler = alt.compiler;
-        Uri uri =
-            new Uri.file('${tmpDir.path}/out.js', windows: Platform.isWindows);
-        validateSourceMap(uri, compiler);
+        Uri mainUri = new Uri.file('${tmpDir.path}/out.js',
+                                   windows: Platform.isWindows);
+        Uri deferredUri = new Uri.file('${tmpDir.path}/out.js_1.part.js',
+                                       windows: Platform.isWindows);
+        validateSourceMap(mainUri, compiler);
+        validateSourceMap(deferredUri, compiler);
 
         print("Deleting '${tmpDir.path}'.");
         tmpDir.deleteSync(recursive: true);
       });
   }));
 }