Remove deprecated sourceMapping syntax.

tests/compiler/dart2js/source_map_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.
 
 // Test that options '--source-map' and '--out' correctly adds
 // `sourceMappingURL` and "file" attributes to source file and source map file.
 
 library test.source_map;
 
 import 'package:async_helper/async_helper.dart';
@@ skipping 31 matching lines @@
   var compiler = compilerFor(SOURCE,
                              showDiagnostics: true,
                              outputProvider: collector,
                              options: options);
   asyncTest(() => compiler.runCompiler(Uri.parse('memory:/main.dart')).then(
       (_) {
     String jsOutput = collector.getOutput('', 'js');
     Expect.isNotNull(jsOutput);
     if (mapping != null) {
       find(jsOutput, '//# sourceMappingURL=$mapping', true);
-      find(jsOutput, '//@ sourceMappingURL=$mapping', true);
     } else {
       find(jsOutput, '//# sourceMappingURL=', false);
-      find(jsOutput, '//@ sourceMappingURL=', false);
     }
     String jsSourceMapOutput = collector.getOutput('', 'js.map');
     Expect.isNotNull(jsSourceMapOutput);
     if (file != null) {
       find(jsSourceMapOutput, '"file": "$file"', true);
     } else {
       find(jsSourceMapOutput, '"file": ', false);
     }
   }));
 }
 
 void main() {
   test();
   test(sourceMap: 'file:/out.js.map');
   test(out: 'file:/out.js');
   test(out: 'file:/out.js', sourceMap: 'file:/out.js.map',
        file: 'out.js', mapping: 'out.js.map');
   test(out: 'file:/dir/out.js', sourceMap: 'file:/dir/out.js.map',
        file: 'out.js', mapping: 'out.js.map');
 }