Disallow generator declarations in certain locations

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index c09eceeae7a6b60d475571908998f8f8545ba590..95baf0a1675e42839e46e13f0892d7d2afd04ad4 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -377,6 +377,8 @@ class CallSite {
     "% loop variable declaration may not have an initializer.")                \
   T(ForInOfLoopMultiBindings,                                                  \
     "Invalid left-hand side in % loop: Must have a single binding.")           \
+  T(GeneratorInLegacyContext,                                                  \
+    "Generator declarations are not allowed in legacy contexts.")              \
   T(IllegalBreak, "Illegal break statement")                                   \
   T(IllegalContinue, "Illegal continue statement")                             \
   T(IllegalLanguageModeDirective,                                              \