Fix Assertions#kotlin(before, after)

fixes #30

src/main/java/org/openrewrite/kotlin/Assertions.java

@@ -46,7 +46,7 @@ public static SourceSpecs kotlin(@Language("kotlin") @Nullable String before, @L
 
     public static SourceSpecs kotlin(@Language("kotlin") @Nullable String before, @Language("kotlin") String after,
                                      Consumer<SourceSpec<K.CompilationUnit>> spec) {
-        SourceSpec<K.CompilationUnit> kotlin = new SourceSpec<>(K.CompilationUnit.class, null, KotlinParser.builder(), before, null);
+        SourceSpec<K.CompilationUnit> kotlin = new SourceSpec<>(K.CompilationUnit.class, null, KotlinParser.builder(), before, s -> after);
         spec.accept(kotlin);
         return kotlin;
     }

src/test/java/org/openrewrite/kotlin/AssertionsTest.java

@@ -0,0 +1,46 @@
+package org.openrewrite.kotlin;
+
+import org.junit.jupiter.api.Test;
+import org.openrewrite.ExecutionContext;
+import org.openrewrite.Issue;
+import org.openrewrite.java.tree.J;
+import org.openrewrite.test.RecipeSpec;
+import org.openrewrite.test.RewriteTest;
+
+import static org.openrewrite.kotlin.Assertions.kotlin;
+import static org.openrewrite.test.RewriteTest.toRecipe;
+
+// This class may be removed after recipes with Assertions are added.
+public class AssertionsTest implements RewriteTest {
+    @Override
+    public void defaults(RecipeSpec spec) {
+        spec.recipe(toRecipe(() -> new KotlinIsoVisitor<>() {
+            @Override
+            public J visitVariable(J.VariableDeclarations.NamedVariable variable, ExecutionContext executionContext) {
+                if ("a".equals(variable.getSimpleName())) {
+                    return variable.withName(variable.getName().withSimpleName("b"));
+                }
+                return variable;
+            }
+        }));
+    }
+
+    @Issue("https://github.com/openrewrite/rewrite-kotlin/issues/30")
+    @Test
+    void isChanged() {
+        rewriteRun(
+          kotlin(
+            """
+                class A {
+                    val a = 1
+                }
+            """,
+            """
+                class A {
+                    val b = 1
+                }
+            """
+          )
+        );
+    }
+}