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

pkg/compiler/lib/src/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 tracer;
 
 import 'dart:async' show EventSink;
 import '../compiler.dart' as api;
 import 'common/work.dart' show
     ItemCompilationContext;
 import 'compiler.dart' show
     Compiler;
-import 'ssa/ssa.dart' as ssa;
+import 'ssa/nodes.dart' as ssa show
+    HGraph;
 import 'ssa/ssa_tracer.dart' show
     HTracer;
 import 'cps_ir/cps_ir_nodes.dart' as cps_ir;
 import 'cps_ir/cps_ir_tracer.dart' show
     IRTracer;
 import 'tree_ir/tree_ir_nodes.dart' as tree_ir;
 import 'tree_ir/tree_ir_tracer.dart' show
     TreeTracer;
 import 'util/util.dart' show
     Indentation;
@@ skipping 93 matching lines @@
   }
 
   void add(String string) {
     output.add(string);
   }
 
   void addIndent() {
     add(_ind.indentation);
   }
 }