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

pkg/compiler/lib/src/resolution/constructors.dart

Index: pkg/compiler/lib/src/resolution/constructors.dart
diff --git a/pkg/compiler/lib/src/resolution/constructors.dart b/pkg/compiler/lib/src/resolution/constructors.dart
index 256be2d3419f99c8432d3db5f17d342ccd54c71e..7b6808fcf4e62ec0c2cbc8d9857b1a76be2ba3a8 100644
--- a/pkg/compiler/lib/src/resolution/constructors.dart
+++ b/pkg/compiler/lib/src/resolution/constructors.dart
@@ -3,6 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.resolution.constructors;
+
+import '../common.dart';
 import '../compiler.dart' show
     Compiler;
 import '../constants/constructors.dart' show
@@ -10,15 +12,6 @@ import '../constants/constructors.dart' show
     RedirectingGenerativeConstantConstructor;
 import '../constants/expressions.dart';
 import '../dart_types.dart';
-import '../diagnostics/diagnostic_listener.dart' show
-    DiagnosticReporter,
-    DiagnosticMessage;
-import '../diagnostics/invariant.dart' show
-    invariant;
-import '../diagnostics/messages.dart' show
-    MessageKind;
-import '../diagnostics/spannable.dart' show
-    Spannable;
 import '../elements/elements.dart';
 import '../elements/modelx.dart' show
     ConstructorElementX,