Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/source/source_class_builder.dart

 // Copyright (c) 2016, 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 fasta.source_class_builder;
 
 import 'package:kernel/ast.dart' show
     Class,
     Constructor,
     Supertype,
     setParents;
 
 import '../errors.dart' show
     internalError,
     inputError;
 
 import '../kernel/kernel_builder.dart' show
     Builder,
     ConstructorReferenceBuilder,
     KernelClassBuilder,
     KernelFieldBuilder,
     KernelFunctionBuilder,
     KernelLibraryBuilder,
     KernelProcedureBuilder,
     KernelTypeBuilder,
     KernelTypeVariableBuilder,
     LibraryBuilder,
-    MemberBuilder,
     MetadataBuilder,
     ProcedureBuilder,
     TypeVariableBuilder;
 
 import '../dill/dill_member_builder.dart' show
     DillMemberBuilder;
 
 class SourceClassBuilder extends KernelClassBuilder {
   final Class cls;
 
@@ skipping 54 matching lines @@
           // TODO(ahe): Report an error if supertype is null.
           cls.implementedTypes.add(supertype);
         }
       }
     }
     return cls;
   }
 
   int convertConstructors(KernelLibraryBuilder library) {
     List<String> oldConstructorNames = <String>[];
-    members.forEach((String name, MemberBuilder builder) {
+    // TODO(sigmund): should be `covariant MemberBuilder`
+    members.forEach((String name, dynamic builder) {
       if (isConstructorName(name, this.name)) {
         oldConstructorNames.add(name);
         String newName = "";
         int index = name.indexOf(".");
         if (index != -1) {
           newName = name.substring(index + 1);
         }
         if (builder is KernelProcedureBuilder) {
           Builder constructor = builder.toConstructor(newName, typeVariables);
           Builder other = members[newName];
@@ skipping 44 matching lines @@
       if (isConstructorName(builder.name, className)) return;
     }
     if (name.indexOf(".") != -1) {
       inputError(null, null, "Only constructors and factories can have names "
           "containing a period ('.'): $name");
     }
     membersInScope[name] = builder;
   });
   return membersInScope;
 }