Do not emit __extends if --noEmitHelpers is set

tests/baselines/reference/noEmitHelpers2.js

@@ -0,0 +1,17 @@
+//// [noEmitHelpers2.ts]
+
+function decorator() { }
+
+@decorator
+class A { }
+
+//// [noEmitHelpers2.js]
+function decorator() { }
+var A = (function () {
+    function A() {
+    }
+    A = __decorate([
+        decorator
+    ], A);
+    return A;
+})();

tests/cases/compiler/noEmitHelpers2.ts

@@ -0,0 +1,10 @@
+// @noemithelpers: true
+// @target: es5
+
+function decorator() { }
+
+@decorator
+class A {
+    constructor(a: number, b: string) {
+    }
+}