Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/source/source_library_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_library_builder;
 
 import 'package:kernel/ast.dart' show
     AsyncMarker,
     ProcedureKind;
 
@@ skipping 36 matching lines @@
     extends LibraryBuilder<T, R> {
   final SourceLoader loader;
 
   final Map<String, Builder> members = <String, Builder>{};
 
   final List<T> types = <T>[];
 
   final List<ConstructorReferenceBuilder> constructorReferences =
       <ConstructorReferenceBuilder>[];
 
-  final List<LibraryBuilder> parts = <LibraryBuilder>[];
+  final List<SourceLibraryBuilder<T, R>> parts = <SourceLibraryBuilder<T, R>>[];
 
   final List<Import> imports = <Import>[];
 
   final Map<String, Builder> exports = <String, Builder>{};
 
   final Scope scope = new Scope(<String, Builder>{}, null, isModifiable: false);
 
   final Uri fileUri;
 
   String name;
@@ skipping 168 matching lines @@
     if (parts.isNotEmpty) {
       internalError("Part with parts: $uri");
     }
     if (exporters.isNotEmpty) {
       internalError(
           "${exporters.first.exporter.uri} attempts to export the part $uri.");
     }
   }
 
   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 "
             "by ${name} ($uri)");
         parts.remove(part);
         return;
       }
       if (part.partOf != name) {
         print("${part.uri} is part of '${part.partOf}' but is used as a part "
             "by '${name}' ($uri)");
@@ skipping 71 matching lines @@
   }
 
   int resolveConstructors(_) {
     int count = 0;
     members.forEach((String name, Builder member) {
       count += member.resolveConstructors(this);
     });
     return count;
   }
 }