Fix recently introduced strong-mode errors

pkg/analyzer/test/source/package_map_resolver_test.dart

Index: pkg/analyzer/test/source/package_map_resolver_test.dart
diff --git a/pkg/analyzer/test/source/package_map_resolver_test.dart b/pkg/analyzer/test/source/package_map_resolver_test.dart
index 7cdcabc4430fa9407e9239fe3d6d034eca2d3251..bcdd5be8e7020223638153c87e9c23de1da121c0 100644
--- a/pkg/analyzer/test/source/package_map_resolver_test.dart
+++ b/pkg/analyzer/test/source/package_map_resolver_test.dart
@@ -62,7 +62,7 @@ class _PackageMapUriResolverTest {
     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/')
       ]
@@ -99,8 +99,8 @@ class _PackageMapUriResolverTest {
     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');
@@ -158,8 +158,8 @@ class _PackageMapUriResolverTest {
     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');
@@ -187,7 +187,7 @@ class _PackageMapUriResolverTest {
     provider.newFile(file2, 'library bar');
     PackageMapUriResolver resolver =
         new PackageMapUriResolver(provider, <String, List<Folder>>{
-      'foo': [
+      'foo': <Folder>[
         provider.getResource('/foo1/lib'),
         provider.getResource('/foo2/lib')
       ]
@@ -211,11 +211,11 @@ class _PackageMapUriResolverTest {
     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')
       ]