fixup! Introduce additional tests for ES6 templates

Ensure test files assert the described behavior. Extend coverage to
strict and non-strict mode explicitly.

test/language/expressions/tagged-template/call-expression-context-no-strict.js

@@ -0,0 +1,22 @@
+// Copyright (C) Copyright 2015 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+es6id: 12.3.7
+description: Invocation context for call expressions
+info: >
+    A tagged template is a function call where the arguments of the call are
+    derived from a TemplateLiteral. The actual arguments include a template
+    object and the values produced by evaluating the expressions embedded
+    within the TemplateLiteral.
+flags: [noStrict]
+---*/
+var context = null;
+var fn = function() {
+  return function() {
+    context = this;
+  };
+};
+
+fn()`NoSubstitutionTemplate`;
+
+assert.sameValue(context, this);

test/language/expressions/tagged-template/call-expression-context-strict.js

@@ -8,14 +8,15 @@ info: >
     derived from a TemplateLiteral. The actual arguments include a template
     object and the values produced by evaluating the expressions embedded
     within the TemplateLiteral.
+flags: [onlyStrict]
 ---*/
-var context;
+var context = null;
 var fn = function() {
   return function() {
     context = this;
   };
 };
 
-fn`NoSubstitutionTemplate`;
+fn()`NoSubstitutionTemplate`;
 
 assert.sameValue(context, undefined);