Add Path#isInStrictMode (#7712)

* Add Path#isInStrictMode

* Fix undefined directives

* Explicitly return for arrow expressions

packages/babel-plugin-transform-classes/src/transformClass.js

@@ -688,26 +688,9 @@ export default function transformClass(
       classState.staticPropBody.map(fn => fn(t.cloneNode(classState.classRef))),
     );
 
-    const strictParent = path.findParent(path => {
-      if (path.isProgram() && path.node.sourceType === "module") {
-        return true;
-      }
-
-      if (path.isClassBody()) {
-        return true;
-      }
-
-      if (!path.isProgram() && !path.isBlockStatement()) {
-        return false;
-      }
-
-      return path.node.directives.some(
-        directive => directive.value.value === "use strict",
-      );
-    });
-
+    const isStrict = path.isInStrictMode();
     let constructorOnly = classState.classId && body.length === 1;
-    if (constructorOnly && !strictParent) {
+    if (constructorOnly && !isStrict) {
       for (const param of classState.construct.params) {
         // It's illegal to put a use strict directive into the body of a function
         // with non-simple parameters for some reason. So, we have to use a strict
@@ -720,7 +703,7 @@ export default function transformClass(
     }
 
     const directives = constructorOnly ? body[0].body.directives : [];
-    if (!strictParent) {
+    if (!isStrict) {
       directives.push(t.directive(t.directiveLiteral("use strict")));
     }