Remove dart:io import from ssa/tracer.dart.

dart/sdk/lib/_internal/compiler/implementation/ssa/tracer.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 tracer;
 
-import 'dart:io';
+import 'dart:async' show EventSink;
+
 import 'ssa.dart';
 import '../js_backend/js_backend.dart';
 import '../dart2jslib.dart';
 
 const bool GENERATE_SSA_TRACE = false;
 const String SSA_TRACE_FILTER = null;
 
 class HTracer extends HGraphVisitor implements Tracer {
   JavaScriptItemCompilationContext context;
   int indent = 0;
-  final RandomAccessFile output;
+  final EventSink<String> output;
   final bool enabled = GENERATE_SSA_TRACE;
   bool traceActive = false;
 
-  HTracer([String path = "dart.cfg"])
-      : output = GENERATE_SSA_TRACE
-          ? new File(path).openSync(mode: FileMode.WRITE)
-          : null;
+  HTracer(this.output);
 
   void close() {
-    if (enabled) output.closeSync();
+    if (enabled) output.close();
   }
 
   void traceCompilation(String methodName,
                         JavaScriptItemCompilationContext compilationContext) {
     if (!enabled) return;
     this.context = compilationContext;
     traceActive =
         SSA_TRACE_FILTER == null || methodName.contains(SSA_TRACE_FILTER);
     if (!traceActive) return;
     tag("compilation", () {
@@ skipping 110 matching lines @@
 
   void printProperty(String propertyName, var value) {
     if (value is num) {
       println("$propertyName $value");
     } else {
       println('$propertyName "$value"');
     }
   }
 
   void add(String string) {
-    output.writeStringSync(string);
+    output.add(string);
   }
 
   void addIndent() {
     for (int i = 0; i < indent; i++) {
       add("  ");
     }
   }
 }
 
 class HInstructionStringifier implements HVisitor<String> {
@@ skipping 382 matching lines @@
 
   String visitTypeConversion(HTypeConversion node) {
     return "TypeConversion: ${temporaryId(node.checkedInput)} to "
       "${node.instructionType}";
   }
 
   String visitRangeConversion(HRangeConversion node) {
     return "RangeConversion: ${node.checkedInput}";
   }
 }