Merge pull request microsoft#33581 from microsoft/fix-lint-20190924

Fix lint on master

Author: sandersn

src/compiler/checker.ts

@@ -33314,6 +33314,7 @@ namespace ts {
 
                 // Modifiers are never allowed on properties except for 'async' on a method declaration
                 if (prop.modifiers) {
+                    // eslint-disable-next-line @typescript-eslint/no-unnecessary-type-assertion
                     for (const mod of prop.modifiers!) { // TODO: GH#19955
                         if (mod.kind !== SyntaxKind.AsyncKeyword || prop.kind !== SyntaxKind.MethodDeclaration) {
                             grammarErrorOnNode(mod, Diagnostics._0_modifier_cannot_be_used_here, getTextOfNode(mod));