Added test for '_this'.

tests/cases/compiler/underscoreThisInDerivedClass01.ts

@@ -0,0 +1,23 @@
+// @target es5
+
+// Original test intent:
+// When arrow functions capture 'this', the lexical 'this' owner
+// currently captures 'this' using a variable named '_this'.
+// That means that '_this' becomes a reserved identifier in certain places.
+//
+// Constructors have adopted the same identifier name ('_this')
+// for capturing any potential return values from super calls,
+// so we expect the same behavior.
+
+class C {
+    constructor() {
+        return {};
+    }
+}
+
+class D extends C {
+    constructor() {
+        var _this = "uh-oh?";
+        super();
+    }
+}