Don't emit '_this' declaration in ES6 or later.

src/compiler/checker.ts

@@ -5582,7 +5582,7 @@ module ts {
                     needToCaptureLexicalThis = false;
                     while (container && container.kind === SyntaxKind.ArrowFunction) {
                         container = getSuperContainer(container, /*includeFunctions*/ true);
-                        needToCaptureLexicalThis = true;
+                        needToCaptureLexicalThis = languageVersion < ScriptTarget.ES6;
                     }
 
                     // topmost container must be something that is directly nested in the class declaration

tests/baselines/reference/computedPropertyNames31_ES6.js

@@ -23,7 +23,6 @@ class Base {
 }
 class C extends Base {
     foo() {
-        var _this = this;
         (() => {
             var obj = {
                 [super.bar()]() { } // needs capture

tests/baselines/reference/emitClassDeclarationWithSuperMethodCall01.js

@@ -18,7 +18,6 @@ class Parent {
 }
 class Foo extends Parent {
     foo() {
-        var _this = this;
         var x = () => super.foo();
     }
 }