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

pkg/compiler/lib/src/helpers/helpers.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 for debugging helpers. The unittest analyze_unused_test checks that
 /// the helper are not used in production code.
 
 library dart2js.helpers;
 
-import 'dart:async' show
-    EventSink;
-import 'dart:collection';
-import 'dart:convert';
-
-import '../../compiler.dart';
 import '../common.dart';
-import '../compiler.dart' show
-    Compiler;
 import '../diagnostics/invariant.dart' show
     DEBUG_MODE;
 import '../util/util.dart';
 
-part 'debug_collection.dart';
-part 'trace.dart';
-part 'expensive_map.dart';
-part 'expensive_set.dart';
-part 'stats.dart';
-part 'track_map.dart';
+import 'trace.dart';
+
+export 'debug_collection.dart';
+export 'trace.dart';
+export 'expensive_map.dart';
+export 'expensive_set.dart';
+export 'stats.dart';
+export 'track_map.dart';
 
 /// Global flag to enable [debugPrint]. This should always be `true` by default
 /// and be set to `false` as a means to temporarily turn off all debugging
 /// printouts.
 const bool DEBUG_PRINT_ENABLED = true;
 
 /// Enables debug mode.
 ///
 /// Sets the [DEBUG_MODE] to `true`.
 void enableDebugMode() {
@@ skipping 103 matching lines @@
       if (printTrace) {
         trace(msg);
       } else {
         debugPrint(msg);
       }
     }
     return true;
   }
   return false;
 }