Reduce strong mode errors and warnings

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

Index: pkg/front_end/lib/src/fasta/source/source_class_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_class_builder.dart b/pkg/front_end/lib/src/fasta/source/source_class_builder.dart
index 1ec438d3eea2861f8c8cb41590b8f88b575624a1..738d2f6fac9368fbd56ac23f1c19aea1eb321c59 100644
--- a/pkg/front_end/lib/src/fasta/source/source_class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_class_builder.dart
@@ -100,7 +100,8 @@ class SourceClassBuilder extends KernelClassBuilder {
 
   int convertConstructors(KernelLibraryBuilder library) {
     List<String> oldConstructorNames = <String>[];
-    members.forEach((String name, MemberBuilder builder) {
+    members.forEach((String name, Builder b) {
+      MemberBuilder builder = b;
       if (isConstructorName(name, this.name)) {
         oldConstructorNames.add(name);
         String newName = "";