Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/kernel/kernel_enum_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.kernel_enum_builder;
 
 import 'package:kernel/ast.dart' show
     Arguments,
     AsyncMarker,
     Class,
@@ skipping 118 matching lines @@
           new KernelFieldBuilder(null, selfType, name, constMask | staticMask);
       members[name] = fieldBuilder;
       toStringEntries.add(new MapEntry(
               new IntLiteral(index), new StringLiteral("$className.$name")));
       index++;
     }
     MapLiteral toStringMap = new MapLiteral(toStringEntries, isConst: true);
     KernelEnumBuilder enumBuilder = new KernelEnumBuilder.internal(metadata,
         name, members, types, cls, constants, toStringMap, intType, stringType,
         parent);
-    members.forEach((String name, MemberBuilder builder) {
+    // TODO(sigmund): dynamic should be `covariant MemberBuilder`.
+    members.forEach((String name, dynamic b) {
+      MemberBuilder builder = b;
       builder.parent = enumBuilder;
     });
     selfType.builder = enumBuilder;
     return enumBuilder;
   }
 
   InterfaceType buildType(List<KernelTypeBuilder> arguments) {
     return cls.rawType;
   }
 
@@ skipping 32 matching lines @@
       Arguments arguments =
           new Arguments(<Expression>[new IntLiteral(index++)]);
       field.initializer =
           new ConstructorInvocation(constructor, arguments, isConst: true);
     }
     return super.build(libraryBuilder);
   }
 
   Builder findConstructorOrFactory(String name) => null;
 }