Improve error message when reserved word is used as identifier

Signed-off-by: Sergey Tychinin <stychinin@bloomberg.net>

src/compiler/diagnosticMessages.json

@@ -1043,6 +1043,10 @@
         "category": "Error",
         "code": 1358
     },
+    "Identifier expected. '{0}' is a reserved word that cannot be used here.": {
+        "category": "Error",
+        "code": 1359
+    },
 
     "The types of '{0}' are incompatible between these types.": {
         "category": "Error",

src/compiler/parser.ts

@@ -1401,7 +1401,14 @@ namespace ts {
             // Only for end of file because the error gets reported incorrectly on embedded script tags.
             const reportAtCurrentPosition = token() === SyntaxKind.EndOfFileToken;
 
-            return createMissingNode<Identifier>(SyntaxKind.Identifier, reportAtCurrentPosition, diagnosticMessage || Diagnostics.Identifier_expected);
+            const isReservedWord = scanner.isReservedWord();
+            const msgArg = scanner.getTokenText();
+
+            const defaultMessage = isReservedWord ?
+                Diagnostics.Identifier_expected_0_is_a_reserved_word_that_cannot_be_used_here :
+                Diagnostics.Identifier_expected;
+
+            return createMissingNode<Identifier>(SyntaxKind.Identifier, reportAtCurrentPosition, diagnosticMessage || defaultMessage, msgArg);
         }
 
         function parseIdentifier(diagnosticMessage?: DiagnosticMessage): Identifier {