Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart
index 825bc27bd5c2427ec9721a42ea7c6e3e21f8f4f3..424c2a2d636155d8874307a47d9c8cd084e9ee21 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart
@@ -136,7 +136,9 @@ class KernelEnumBuilder extends SourceClassBuilder
     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;