Use early bail-out for contextual identifier checks.

Author: DanielRosenwasser

src/compiler/binder.ts

@@ -2419,9 +2419,13 @@ function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
 
             // strict mode identifiers
             const originalKeywordKind = identifierToKeywordKind(node);
+            if (originalKeywordKind === undefined) {
+                return;
+            }
+
             if (inStrictMode &&
-                originalKeywordKind! >= SyntaxKind.FirstFutureReservedWord &&
-                originalKeywordKind! <= SyntaxKind.LastFutureReservedWord) {
+                originalKeywordKind >= SyntaxKind.FirstFutureReservedWord &&
+                originalKeywordKind <= SyntaxKind.LastFutureReservedWord) {
                 file.bindDiagnostics.push(createDiagnosticForNode(node,
                     getStrictModeIdentifierMessage(node), declarationNameToString(node)));
             }