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

pkg/compiler/lib/src/ssa/ssa.dart

 // Copyright (c) 2012, 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 ssa;
 
 import 'dart:collection';
 
 import 'package:js_runtime/shared/embedded_names.dart';
 
 import '../closure.dart';
+import '../common.dart';
 import '../common/codegen.dart' show
     CodegenRegistry,
     CodegenWorkItem;
 import '../common/names.dart' show
     Identifiers,
     Selectors;
 import '../common/resolution.dart' show
     Resolution;
 import '../common/tasks.dart' show
     CompilerTask;
 import '../compiler.dart' show
     Compiler;
 import '../constant_system_dart.dart';
 import '../constants/constant_system.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../dart_types.dart';
-import '../diagnostics/diagnostic_listener.dart' show
-    DiagnosticReporter;
-import '../diagnostics/invariant.dart' show
-    invariant;
-import '../diagnostics/messages.dart';
-import '../diagnostics/spannable.dart' show
-    CURRENT_ELEMENT_SPANNABLE,
-    Spannable;
+import '../diagnostics/messages.dart' show
+    Message,
+    MessageTemplate;
 import '../elements/elements.dart';
 import '../elements/modelx.dart' show
     ConstructorBodyElementX,
     ElementX,
     VariableElementX;
 import '../io/source_information.dart';
 import '../js/js.dart' as js;
 import '../js_backend/js_backend.dart';
 import '../js_emitter/js_emitter.dart' show
     CodeEmitterTask,
@@ skipping 28 matching lines @@
 part 'interceptor_simplifier.dart';
 part 'invoke_dynamic_specializers.dart';
 part 'nodes.dart';
 part 'optimize.dart';
 part 'types.dart';
 part 'types_propagation.dart';
 part 'validate.dart';
 part 'variable_allocator.dart';
 part 'value_range_analyzer.dart';
 part 'value_set.dart';