dart2js cleanup: remove unused imports and variables

pkg/compiler/lib/src/tree_ir/tree_ir_tracer.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.
 
 library tree_ir_tracer;
 
 import 'dart:async' show EventSink;
 import '../tracer.dart';
 import 'tree_ir_nodes.dart';
 
@@ skipping 569 matching lines @@
     String value = visitExpression(node.value);
     return 'SetIndex($object, $index, $value)';
   }
 
   @override
   String visitAwait(Await node) {
     String value = visitExpression(node.input);
     return 'Await($value)';
   }
 
-  @override
   String visitYield(Yield node) {
     String value = visitExpression(node.input);
     return 'Yield($value)';
   }
 }
 
 /**
  * Invents (and remembers) names for Variables that do not have an associated
  * identifier.
  *
@@ skipping 11 matching lines @@
       String prefix = v.element == null ? 'v' : '${v.element.name}_';
       while (name == null || _usedNames.contains(name)) {
         name = "$prefix${_counter++}";
       }
       _names[v] = name;
       _usedNames.add(name);
     }
     return name;
   }
 }