Fix all strong mode warnings in protoc-plugin

lib/extension_generator.dart

 // Copyright (c) 2013, 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.
 
 part of protoc;
 
 class ExtensionGenerator {
   final FieldDescriptorProto _descriptor;
   final ProtobufContainer _parent;
 
@@ skipping 37 matching lines @@
       // The type of this extension is defined in a different file,
       // so we need to import it.
       imports.add(typeGen.fileGen);
     }
   }
 
   void generate(IndentingWriter out) {
     if (_field == null) throw new StateError("resolve not called");
 
     String name = _field.dartFieldName;
+    var type = _field.baseType;
+    var dartType = type.getDartType(package);
 
     if (_field.isRepeated) {
       out.print('static final Extension $name = '
-          'new Extension.repeated(\'$_extendedClassName\', \'$name\', '
-          '${_field.number}, ${_field.typeConstant}');
-      var type = _field.baseType;
+          'new Extension<$dartType>.repeated(\'$_extendedClassName\','
+          ' \'$name\', ${_field.number}, ${_field.typeConstant}');
       if (type.isMessage || type.isGroup) {
-        var dartClass = type.getDartType(package);
-        out.println(', $dartClass.$checkItem, $dartClass.create);');
+        out.println(', $dartType.$checkItem, $dartType.create);');
       } else if (type.isEnum) {
-        var dartClass = type.getDartType(package);
-        out.println(', $dartClass.$checkItem, null, $dartClass.valueOf);');
+        out.println(', $dartType.$checkItem, null, $dartType.valueOf);');
       } else {
         out.println(", getCheckFunction(${_field.typeConstant}));");
       }
       return;
     }
 
     out.print('static final Extension $name = '
-        'new Extension(\'$_extendedClassName\', \'$name\', '
+        'new Extension<$dartType>(\'$_extendedClassName\', \'$name\', '
         '${_field.number}, ${_field.typeConstant}');
 
     String initializer = _field.generateDefaultFunction(package);
 
-    var type = _field.baseType;
     if (type.isMessage || type.isGroup) {
-      var dartClass = type.getDartType(package);
-      out.println(', $initializer, $dartClass.create);');
+      out.println(', $initializer, $dartType.create);');
     } else if (type.isEnum) {
       var dartEnum = type.getDartType(package);
       String valueOf = '(var v) => $dartEnum.valueOf(v)';
       out.println(", $initializer, null, $valueOf);");
     } else if (initializer != null) {
       out.println(", $initializer);");
     } else {
       out.println(");");
     }
   }
 }