Report a semantic error for an arrow function with a "this" parameter.

Fixes #9744.

src/compiler/checker.ts

@@ -21857,6 +21857,9 @@ namespace ts {
                 if (func.kind === SyntaxKind.Constructor || func.kind === SyntaxKind.ConstructSignature || func.kind === SyntaxKind.ConstructorType) {
                     error(node, Diagnostics.A_constructor_cannot_have_a_this_parameter);
                 }
+                if (func.kind === SyntaxKind.ArrowFunction) {
+                    error(node, Diagnostics.An_arrow_function_cannot_have_a_this_parameter);
+                }
             }
 
             // Only check rest parameter type if it's not a binding pattern. Since binding patterns are

src/compiler/diagnosticMessages.json

@@ -2417,6 +2417,10 @@
         "category": "Error",
         "code": 2729
     },
+    "An arrow function cannot have a 'this' parameter.": {
+        "category": "Error",
+        "code": 2730
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

src/compiler/parser.ts

@@ -3537,8 +3537,9 @@ namespace ts {
                 }
 
                 // If we had "(" followed by something that's not an identifier,
-                // then this definitely doesn't look like a lambda.
-                if (!isIdentifier()) {
+                // then this definitely doesn't look like a lambda.  "this" is not
+                // valid, but we want to parse it and then give a semantic error.
+                if (!isIdentifier() && second !== SyntaxKind.ThisKeyword) {
                     return Tristate.False;
                 }