Always wrap classes with decorators or static properties in an IIFE

Currently only script targets less than or equal to ES5 will wrap classes.  However, the wrapping is also crucial to file size optimizations for ES2015+ as well.  Without the IIFE wrapper, minification tools do not elide the class.  This is due to references to the class being present within the downlevelled decorator and static property code.
This change represents the full completion of issue #15857

Author: clydin

src/compiler/transformers/ts.ts

@@ -23,11 +23,10 @@ namespace ts {
         IsNamedExternalExport = 1 << 4,
         IsDefaultExternalExport = 1 << 5,
         IsDerivedClass = 1 << 6,
-        UseImmediatelyInvokedFunctionExpression = 1 << 7,
 
         HasAnyDecorators = HasConstructorDecorators | HasMemberDecorators,
         NeedsName = HasStaticInitializedProperties | HasMemberDecorators,
-        MayNeedImmediatelyInvokedFunctionExpression = HasAnyDecorators | HasStaticInitializedProperties,
+        UseImmediatelyInvokedFunctionExpression = HasAnyDecorators | HasStaticInitializedProperties,
         IsExported = IsExportOfNamespace | IsDefaultExternalExport | IsNamedExternalExport,
     }
 
@@ -590,7 +589,6 @@ namespace ts {
             if (isExportOfNamespace(node)) facts |= ClassFacts.IsExportOfNamespace;
             else if (isDefaultExternalModuleExport(node)) facts |= ClassFacts.IsDefaultExternalExport;
             else if (isNamedExternalModuleExport(node)) facts |= ClassFacts.IsNamedExternalExport;
-            if (languageVersion <= ScriptTarget.ES5 && (facts & ClassFacts.MayNeedImmediatelyInvokedFunctionExpression)) facts |= ClassFacts.UseImmediatelyInvokedFunctionExpression;
             return facts;
         }
 
@@ -661,6 +659,12 @@ namespace ts {
                 const iife = createImmediatelyInvokedArrowFunction(statements);
                 setEmitFlags(iife, EmitFlags.TypeScriptClassWrapper);
 
+                // Class comment is already added by the ES2015 transform when targeting ES5 or below.
+                // Only add if targetting ES2015+ to prevent duplicates
+                if (languageVersion > ScriptTarget.ES5) {
+                    addSyntheticLeadingComment(iife, SyntaxKind.MultiLineCommentTrivia, "* @class ");
+                }
+
                 const varStatement = createVariableStatement(
                     /*modifiers*/ undefined,
                     createVariableDeclarationList([
@@ -669,7 +673,7 @@ namespace ts {
                             /*type*/ undefined,
                             iife
                         )
-                    ])
+                    ], languageVersion > ScriptTarget.ES5 ? NodeFlags.Let : undefined)
                 );
 
                 setOriginalNode(varStatement, node);