Cleanup 1: split parts into their own libraries, remove unused imports

pkg/compiler/lib/src/ssa/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 ssa.tracer;
 
-import 'dart:async' show
-  EventSink;
+import 'dart:async' show EventSink;
 
-import 'ssa.dart';
-import '../common.dart';
-import '../compiler.dart' show
-    Compiler;
-import '../diagnostics/invariant.dart' show
-    DEBUG_MODE;
+import '../compiler.dart' show Compiler;
+import '../diagnostics/invariant.dart' show DEBUG_MODE;
 import '../js_backend/js_backend.dart';
 import '../tracer.dart';
 
+import 'nodes.dart';
+
 /**
  * Outputs SSA code in a format readable by Hydra IR.
  * Tracing is disabled by default, see ../tracer.dart for how
  * to enable it.
  */
 class HTracer extends HGraphVisitor with TracerUtil {
   Compiler compiler;
   JavaScriptItemCompilationContext context;
   final EventSink<String> output;
 
@@ skipping 503 matching lines @@
   }
 
   String visitAwait(HAwait node) {
     return "await ${temporaryId(node.inputs[0])}";
   }
 
   String visitYield(HYield node) {
     return "yield${node.hasStar ? "*" : ""} ${temporaryId(node.inputs[0])}";
   }
 }