Fix recently introduced strong-mode errors

pkg/analyzer/test/source/package_map_resolver_test.dart

 // Copyright (c) 2014, 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 analyzer.test.source.package_map_resolver_test;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/generated/source.dart';
@@ skipping 44 matching lines @@
     }, throws);
   }
 
   void test_resolve_multiple_folders() {
     const pkgFileA = '/part1/lib/libA.dart';
     const pkgFileB = '/part2/lib/libB.dart';
     provider.newFile(pkgFileA, 'library lib_a');
     provider.newFile(pkgFileB, 'library lib_b');
     PackageMapUriResolver resolver =
         new PackageMapUriResolver(provider, <String, List<Folder>>{
-      'pkg': [
+      'pkg': <Folder>[
         provider.getResource('/part1/lib/'),
         provider.getResource('/part2/lib/')
       ]
     });
     {
       Uri uri = Uri.parse('package:pkg/libA.dart');
       Source result = resolver.resolveAbsolute(uri);
       expect(result, isNotNull);
       expect(result.exists(), isTrue);
       expect(result.uriKind, UriKind.PACKAGE_URI);
@@ skipping 16 matching lines @@
     expect(result, isNull);
   }
 
   void test_resolve_OK() {
     const pkgFileA = '/pkgA/lib/libA.dart';
     const pkgFileB = '/pkgB/lib/libB.dart';
     provider.newFile(pkgFileA, 'library lib_a;');
     provider.newFile(pkgFileB, 'library lib_b;');
     PackageMapUriResolver resolver =
         new PackageMapUriResolver(provider, <String, List<Folder>>{
-      'pkgA': [provider.getResource('/pkgA/lib/')],
-      'pkgB': [provider.getResource('/pkgB/lib/')]
+      'pkgA': <Folder>[provider.getResource('/pkgA/lib/')],
+      'pkgB': <Folder>[provider.getResource('/pkgB/lib/')]
     });
     {
       Uri uri = Uri.parse('package:pkgA/libA.dart');
       Source result = resolver.resolveAbsolute(uri);
       expect(result, isNotNull);
       expect(result.exists(), isTrue);
       expect(result.uriKind, UriKind.PACKAGE_URI);
       expect(result.fullName, pkgFileA);
     }
     {
@@ skipping 37 matching lines @@
     expect(result.uri.toString(), 'package:analyzer/analyzer.dart');
   }
 
   void test_restoreAbsolute() {
     const pkgFileA = '/pkgA/lib/libA.dart';
     const pkgFileB = '/pkgB/lib/src/libB.dart';
     provider.newFile(pkgFileA, 'library lib_a;');
     provider.newFile(pkgFileB, 'library lib_b;');
     PackageMapUriResolver resolver =
         new PackageMapUriResolver(provider, <String, List<Folder>>{
-      'pkgA': [provider.getResource('/pkgA/lib/')],
-      'pkgB': [provider.getResource('/pkgB/lib/')]
+      'pkgA': <Folder>[provider.getResource('/pkgA/lib/')],
+      'pkgB': <Folder>[provider.getResource('/pkgB/lib/')]
     });
     {
       Source source = _createFileSource('/pkgA/lib/libA.dart');
       Uri uri = resolver.restoreAbsolute(source);
       expect(uri, isNotNull);
       expect(uri.toString(), 'package:pkgA/libA.dart');
     }
     {
       Source source = _createFileSource('/pkgB/lib/src/libB.dart');
       Uri uri = resolver.restoreAbsolute(source);
       expect(uri, isNotNull);
       expect(uri.toString(), 'package:pkgB/src/libB.dart');
     }
     {
       Source source = _createFileSource('/no/such/file');
       Uri uri = resolver.restoreAbsolute(source);
       expect(uri, isNull);
     }
   }
 
   void test_restoreAbsolute_ambiguous() {
     const file1 = '/foo1/lib/bar.dart';
     const file2 = '/foo2/lib/bar.dart';
     provider.newFile(file1, 'library bar');
     provider.newFile(file2, 'library bar');
     PackageMapUriResolver resolver =
         new PackageMapUriResolver(provider, <String, List<Folder>>{
-      'foo': [
+      'foo': <Folder>[
         provider.getResource('/foo1/lib'),
         provider.getResource('/foo2/lib')
       ]
     });
     // Restoring file1 should yield a package URI, and that package URI should
     // resolve back to file1.
     Source source1 = _createFileSource(file1);
     Uri uri1 = resolver.restoreAbsolute(source1);
     expect(uri1.toString(), 'package:foo/bar.dart');
     expect(resolver.resolveAbsolute(uri1).fullName, file1);
     // Restoring file2 should not yield a package URI, because there is no URI
     // that resolves to file2.
     Source source2 = _createFileSource(file2);
     expect(resolver.restoreAbsolute(source2), isNull);
   }
 
   void test_restoreAbsolute_longestMatch() {
     const file1 = '/foo1/bar1/lib.dart';
     const file2 = '/foo2/bar2/lib.dart';
     provider.newFile(file1, 'library lib');
     provider.newFile(file2, 'library lib');
     PackageMapUriResolver resolver =
         new PackageMapUriResolver(provider, <String, List<Folder>>{
-      'pkg1': [
+      'pkg1': <Folder>[
         provider.getResource('/foo1'),
         provider.getResource('/foo2/bar2')
       ],
-      'pkg2': [
+      'pkg2': <Folder>[
         provider.getResource('/foo1/bar1'),
         provider.getResource('/foo2')
       ]
     });
     // Restoring file1 should yield a package URI for pkg2, since pkg2's match
     // for the file path (/foo1/bar1) is longer than pkg1's match (/foo1).
     Source source1 = _createFileSource(file1);
     Uri uri1 = resolver.restoreAbsolute(source1);
     expect(uri1.toString(), 'package:pkg2/lib.dart');
     // Restoring file2 should yield a package URI for pkg1, since pkg1's match
     // for the file path (/foo2/bar2) is longer than pkg2's match (/foo2).
     Source source2 = _createFileSource(file2);
     Uri uri2 = resolver.restoreAbsolute(source2);
     expect(uri2.toString(), 'package:pkg1/lib.dart');
   }
 
   Source _createFileSource(String path) {
     return new NonExistingSource(path, toUri(path), UriKind.FILE_URI);
   }
 }