Fix tracer type.

lib/compiler/implementation/ssa/tracer.dart

 // Copyright (c) 2012, 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('tracer');
 
 #import('dart:io');
 #import('ssa.dart');
+#import('../js_backend/js_backend.dart');
 #import('../leg.dart');
 
 final bool GENERATE_SSA_TRACE = false;
 final String SSA_TRACE_FILTER = null;
 
 class HTracer extends HGraphVisitor implements Tracer {
   JavaScriptItemCompilationContext context;
   int indent = 0;
   final RandomAccessFile output;
   final bool enabled = GENERATE_SSA_TRACE;
@@ skipping 494 matching lines @@
 
   String visitIs(HIs node) {
     String type = node.typeExpression.toString();
     return "TypeTest: ${temporaryId(node.expression)} is $type";
   }
 
   String visitTypeConversion(HTypeConversion node) {
     return "TypeConversion: ${temporaryId(node.inputs[0])} to ${node.type}";
   }
 }