Extract a shared J2clSourceUtils class

This just centralizes the checks for whether we're in a J2CL source file as this check was duplicated in multiple places.

PiperOrigin-RevId: 893214710

Author: kevinoconnor7

src/com/google/javascript/jscomp/CheckSideEffects.java

@@ -232,7 +232,8 @@ public void visit(NodeTraversal t, Node n, Node parent) {
 
         if (target.getString().equals(J2CL_PROTECTOR_FN)) {
           // Do not let non-J2CL code abuse it.
-          checkState(n.getSourceFileName().endsWith(".java.js"), "Only allowed for J2CL code");
+          checkState(
+              J2clSourceUtils.isJ2clSource(n.getSourceFileName()), "Only allowed for J2CL code");
           NodeUtil.deleteFunctionCall(n, compiler);
           return;
         }

src/com/google/javascript/jscomp/J2clSourceFileChecker.java

@@ -30,7 +30,7 @@ final class J2clSourceFileChecker implements CompilerPass {
   private static boolean hasJ2cl(Node root) {
     for (Node script = root.getFirstChild(); script != null; script = script.getNext()) {
       checkState(script.isScript());
-      if (script.getSourceFileName() != null && script.getSourceFileName().endsWith(".java.js")) {
+      if (J2clSourceUtils.isJ2clSource(script)) {
         return true;
       }
     }

src/com/google/javascript/jscomp/J2clSourceUtils.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2026 The Closure Compiler Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.javascript.jscomp;
+
+import static com.google.common.base.Preconditions.checkArgument;
+
+import com.google.javascript.rhino.Node;
+
+final class J2clSourceUtils {
+
+  public static boolean isJ2clSource(String sourceFileName) {
+    return sourceFileName != null && sourceFileName.endsWith(".java.js");
+  }
+
+  public static boolean isJ2clSource(Node scriptNode) {
+    checkArgument(scriptNode.isScript());
+    return isJ2clSource(scriptNode.getSourceFileName());
+  }
+
+  public static boolean isInsideJ2clSource(Node node) {
+    Node scriptNode = node.isScript() ? node : NodeUtil.getEnclosingScript(node);
+    if (scriptNode == null) {
+      return false;
+    }
+    return isJ2clSource(scriptNode);
+  }
+
+  private J2clSourceUtils() {}
+}

src/com/google/javascript/jscomp/J2clSuppressWarningsGuard.java

@@ -54,7 +54,7 @@ public final class J2clSuppressWarningsGuard extends WarningsGuard {
 
   @Override
   public @Nullable CheckLevel level(JSError error) {
-    if (error.sourceName() == null || !error.sourceName().endsWith(".java.js")) {
+    if (!J2clSourceUtils.isJ2clSource(error.sourceName())) {
       return null;
     }
 

src/com/google/javascript/jscomp/TypeCheck.java

@@ -557,8 +557,7 @@ public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
         // Errors in type summary files are suppressed, so no use traversing them.
         return false;
       }
-      String filename = n.getSourceFileName();
-      if (filename != null && filename.endsWith(".java.js")) {
+      if (J2clSourceUtils.isJ2clSource(n)) {
         this.subtypingMode = SubtypingMode.IGNORE_NULL_UNDEFINED;
       } else {
         this.subtypingMode = SubtypingMode.NORMAL;