[esnext] implement frontend changes for async/await proposal

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index da9637b9410c18ebfe6a8bd5cec5d8303e04d94a..1cb28adcd9650e8aa569592b9adfc9369d437e65 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -442,6 +442,10 @@ class CallSite {
   T(TemplateOctalLiteral,                                                      \
     "Octal literals are not allowed in template strings.")                     \
   T(ThisFormalParameter, "'this' is not a valid formal parameter name")        \
+  T(AwaitBindingIdentifier,                                                    \
+    "'await' is not a valid identifier name in an async function")             \
+  T(AwaitExpressionFormalParameter,                                            \
+    "Illegal await-expression in formal parameters of async function")         \
   T(TooManyArguments,                                                          \
     "Too many arguments in function call (only 65535 allowed)")                \
   T(TooManyParameters,                                                         \