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

pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart

Index: pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
index e5cd0e658130e8b395f26b7fd9cbe0946b123411..5eddb3784787bc3778b6537df7c6a1a7236e27f6 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
@@ -4,14 +4,12 @@
 
 library tree_ir_builder;
 
-import '../diagnostics/invariant.dart' show
-    InternalErrorFunction;
-import '../diagnostics/spannable.dart' show
-    CURRENT_ELEMENT_SPANNABLE;
-import '../elements/elements.dart';
+import '../common.dart';
+import '../constants/values.dart';
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
+import '../elements/elements.dart';
+
 import 'tree_ir_nodes.dart';
-import '../constants/values.dart';
 
 typedef Statement NodeCallback(Statement next);