Use common.dart to re-export commonly used entities.

pkg/compiler/lib/src/tree_ir/tree_ir_builder.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 tree_ir_builder;
 
-import '../diagnostics/invariant.dart' show
-    InternalErrorFunction;
-import '../diagnostics/spannable.dart' show
-    CURRENT_ELEMENT_SPANNABLE;
+import '../common.dart';
+import '../constants/values.dart';
+import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
 import '../elements/elements.dart';
-import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
+
 import 'tree_ir_nodes.dart';
-import '../constants/values.dart';
 
 typedef Statement NodeCallback(Statement next);
 
 /**
  * Builder translates from CPS-based IR to direct-style Tree.
  *
  * A call `Invoke(fun, cont, args)`, where cont is a singly-referenced
  * non-exit continuation `Cont(v, body)` is translated into a direct-style call
  * whose value is bound in the continuation body:
  *
@@ skipping 674 matching lines @@
     internalError(CURRENT_ELEMENT_SPANNABLE, 'Unexpected IR node: $node');
   }
 
   visitFunctionDefinition(cps_ir.FunctionDefinition node) {
     unexpectedNode(node);
   }
   visitParameter(cps_ir.Parameter node) => unexpectedNode(node);
   visitContinuation(cps_ir.Continuation node) => unexpectedNode(node);
   visitMutableVariable(cps_ir.MutableVariable node) => unexpectedNode(node);
 }