Finish strong mode cleaning of dart2js.

pkg/compiler/lib/src/js_emitter/full_emitter/class_builder.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.
 
 library dart2js.js_emitter.full_emitter.class_builder;
 
 import '../../elements/entities.dart';
 import '../../js/js.dart' as jsAst;
 import '../../js/js.dart' show js;
 import '../../js_backend/js_backend.dart' show Namer;
 
 /**
  * A data structure for collecting fragments of a class definition.
  */
 class ClassBuilder {
   final List<jsAst.Property> properties = <jsAst.Property>[];
   final List<jsAst.Literal> fields = <jsAst.Literal>[];
 
   jsAst.Name superName;
   jsAst.Node functionType;
-  List<jsAst.Node> fieldMetadata;
+  List<jsAst.Expression> fieldMetadata;

[ahe, 2017/06/15 09:36:39]

I'm concerned about causing a checked mode problem.

[Johnni Winther, 2017/06/15 10:21:26]

If you change `var fieldMetadata = [];` in class_emitter line 128 to ` var
fieldMetadata = <Expression>[];` it should be fine.

[ahe, 2017/06/15 11:41:57]

Done.

 
   final Entity element;
   final Namer namer;
   final bool isForActualClass;
 
   ClassBuilder.forLibrary(LibraryEntity library, this.namer)
       : isForActualClass = false,
         element = library;
 
   ClassBuilder.forClass(ClassEntity cls, this.namer)
@@ skipping 36 matching lines @@
         if (functionType != null) {
           // See [functionTypeEncodingDescription] above.
           parts.add(js.stringPart(':'));
           parts.add(functionType);
         }
       }
       parts.add(js.stringPart(';'));
     }
     // See [fieldEncodingDescription] above.
     parts.addAll(js.joinLiterals(fields, js.stringPart(',')));
-    var classData = js.concatenateStrings(parts, addQuotes: true);
+    dynamic classData = js.concatenateStrings(parts, addQuotes: true);
     if (fieldMetadata != null) {
       // If we need to store fieldMetadata, classData is turned into an array,
       // and the field metadata is appended. So if classData is just a string,
       // there is no field metadata.
       classData =
           new jsAst.ArrayInitializer([classData]..addAll(fieldMetadata));
     }
     List<jsAst.Property> fieldsAndProperties;
     if (emitClassDescriptor) {
       fieldsAndProperties = <jsAst.Property>[];
       fieldsAndProperties.add(new jsAst.Property(
           js.string(namer.classDescriptorProperty), classData));
       fieldsAndProperties.addAll(properties);
     } else {
       fieldsAndProperties = properties;
     }
     return new jsAst.ObjectInitializer(fieldsAndProperties, isOneLiner: false);
   }
 }