Implement ES2015 labelled function declaration restrictions

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index dac623fa8f3199c5fad858775e5b5bced0fcd50a..300030832a2a00cca632c85a1d033eb87131aa76 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -398,6 +398,9 @@ class CallSite {
   T(JsonParseUnexpectedTokenNumber, "Unexpected number in JSON at position %") \
   T(JsonParseUnexpectedTokenString, "Unexpected string in JSON at position %") \
   T(LabelRedeclaration, "Label '%' has already been declared")                 \
+  T(LabelledFunctionDeclaration,                                               \
+    "Labelled function declaration not allowed as the body of a control flow " \
+    "structure")                                                               \
   T(MalformedArrowFunParamList, "Malformed arrow function parameter list")     \
   T(MalformedRegExp, "Invalid regular expression: /%/: %")                     \
   T(MalformedRegExpFlags, "Invalid regular expression flags")                  \