Report an undefined variable error for "() => y" in VarCheck

Previously VarCheck was allowing anything that was the child of a function expression to be an undeclared name, not just the function name. Now it correctly reports an error for "() => y" and adds the undeclared name "y" to the externs.

Fixes #2843

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188941589

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

@@ -166,7 +166,7 @@ public void hotSwapScript(Node scriptRoot, Node originalRoot) {
 
   @Override
   public void visit(NodeTraversal t, Node n, Node parent) {
-    if (n.isName() || (n.isStringKey() && !n.hasChildren())) {
+    if (n.isName()) {
       String varName = n.getString();
 
       // Only a function can have an empty name.
@@ -200,8 +200,8 @@ public void visit(NodeTraversal t, Node n, Node parent) {
       Scope scope = t.getScope();
       Var var = scope.getVar(varName);
       if (var == null) {
-        if (NodeUtil.isFunctionExpression(parent)
-            || (NodeUtil.isClassExpression(parent) && n == parent.getFirstChild())) {
+        if ((NodeUtil.isFunctionExpression(parent) || NodeUtil.isClassExpression(parent))
+            && n == parent.getFirstChild()) {
           // e.g. [ function foo() {} ], it's okay if "foo" isn't defined in the
           // current scope.
         } else if (NodeUtil.isNonlocalModuleExportName(n)) {

test/com/google/javascript/jscomp/VarCheckTest.java

@@ -104,6 +104,10 @@ public void testReferencedVarNotDefined() {
     testError("x = 0;", VarCheck.UNDEFINED_VAR_ERROR);
   }
 
+  public void testReferencedVarNotDefined_arrowFunctionBody() {
+    testError("() => y", VarCheck.UNDEFINED_VAR_ERROR);
+  }
+
   public void testReferencedLetNotDefined() {
     testError("{ let x = 1; } var y = x;", VarCheck.UNDEFINED_VAR_ERROR);
   }