Reduce strong mode errors and warnings

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

Index: pkg/front_end/lib/src/fasta/source/source_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
index 934b98092526ce2a7b811d3fbff0ea7dd465f94b..182b2cbdf01e3d0bcfcb394a6310157279db312b 100644
--- a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
@@ -54,7 +54,7 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
   final List<ConstructorReferenceBuilder> constructorReferences =
       <ConstructorReferenceBuilder>[];
 
-  final List<LibraryBuilder> parts = <LibraryBuilder>[];
+  final List<SourceLibraryBuilder<T, R>> parts = <SourceLibraryBuilder<T, R>>[];
 
   final List<Import> imports = <Import>[];
 
@@ -243,12 +243,12 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
   }
 
   void includeParts() {
-    for (LibraryBuilder part in parts.toList()) {
+    for (SourceLibraryBuilder<T, R> part in parts.toList()) {
       includePart(part);
     }
   }
 
-  void includePart(SourceLibraryBuilder part) {
+  void includePart(SourceLibraryBuilder<T, R> part) {
     if (name != null) {
       if (part.partOf == null) {
         print("${part.uri} has no 'part of' declaration but is used as a part "