Enforce and fix prefer_const_constructors

diff --git a/pkgs/stream_transform/analysis_options.yaml b/pkgs/stream_transform/analysis_options.yaml
index 20e06b2..906ceb0 100644
--- a/pkgs/stream_transform/analysis_options.yaml
+++ b/pkgs/stream_transform/analysis_options.yaml

@@ -59,7 +59,7 @@
     - prefer_adjacent_string_concatenation
     - prefer_collection_literals
     - prefer_conditional_assignment
-    #- prefer_const_constructors
+    - prefer_const_constructors
     - prefer_contains
     - prefer_equal_for_default_values
     - prefer_final_fields

diff --git a/pkgs/stream_transform/test/combine_latest_test.dart b/pkgs/stream_transform/test/combine_latest_test.dart
index e3987c7..f8156bc 100644
--- a/pkgs/stream_transform/test/combine_latest_test.dart
+++ b/pkgs/stream_transform/test/combine_latest_test.dart

@@ -92,7 +92,7 @@
 
     test('ends if source stream closes without ever emitting a value',
         () async {
-      var source = Stream<int>.empty();
+      var source = const Stream<int>.empty();
       var other = StreamController<int>();
 
       int sum(int a, int b) => a + b;
@@ -109,7 +109,7 @@
 
     test('ends if other stream closes without ever emitting a value', () async {
       var source = StreamController<int>();
-      var other = Stream<int>.empty();
+      var other = const Stream<int>.empty();
 
       int sum(int a, int b) => a + b;
 
@@ -178,4 +178,4 @@
 }
 
 Matcher _isException(int id) =>
-    TypeMatcher<_NumberedException>().having((n) => n.id, 'id', id);
+    const TypeMatcher<_NumberedException>().having((n) => n.id, 'id', id);

diff --git a/pkgs/stream_transform/test/scan_test.dart b/pkgs/stream_transform/test/scan_test.dart
index 71472a6..6c4f41a 100644
--- a/pkgs/stream_transform/test/scan_test.dart
+++ b/pkgs/stream_transform/test/scan_test.dart

@@ -57,7 +57,8 @@
             .transform(scan<int, Future<int>>(Future.value(0), sum))
             .toList();
 
-        expect(result, [TypeMatcher<Future>(), TypeMatcher<Future>()]);
+        expect(
+            result, [const TypeMatcher<Future>(), const TypeMatcher<Future>()]);
         expect(await Future.wait(result), [1, 3]);
       });