Emit Object.assign for spread for targets >= ES6

src/compiler/transformers/esnext.ts

@@ -402,7 +402,7 @@ namespace ts {
     };
 
     export function createAssignHelper(context: TransformationContext, attributesSegments: Expression[]) {
-        if (context.getCompilerOptions().target === ScriptTarget.ES2015) {
+        if (context.getCompilerOptions().target >= ScriptTarget.ES2015) {
             return createCall(createPropertyAccess(createIdentifier("Object"), "assign"),
                               /*typeArguments*/ undefined,
                               attributesSegments);