Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/source/source_loader.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_loader;
 
 import 'dart:async' show
     Future;
 
 import 'dart:io' show
@@ skipping 179 matching lines @@
     builders.forEach((Uri uri, LibraryBuilder library) {
       if (library is SourceLibraryBuilder) {
         library.addImportsToScope();
       }
     });
     ticker.logMs("Computed library scopes");
     // debugPrintExports();
   }
 
   void debugPrintExports() {
-    builders.forEach((Uri uri, SourceLibraryBuilder library) {
+    // TODO(sigmund): should be `covarint SourceLibraryBuilder`.
+    builders.forEach((Uri uri, dynamic l) {
+      SourceLibraryBuilder library = l;
       Set<Builder> members = new Set<Builder>();
       library.members.forEach((String name, Builder member) {
         while (member != null) {
           members.add(member);
           member = member.next;
         }
       });
       List<String> exports = <String>[];
       library.exports.forEach((String name, Builder member) {
         while (member != null) {
@@ skipping 135 matching lines @@
     ticker.logMs("Built analyzer element model.");
   }
 
   void computeHierarchy(Program program) {
     hierarchy = new ClassHierarchy(program);
     ticker.logMs("Computed class hierarchy");
     coreTypes = new CoreTypes(program);
     ticker.logMs("Computed core types");
   }
 }