Don't pass a null context to Function.prototype.call.

This prevents PhantomJS tests from hanging in the open-source React repo.

Until the advent of `"use strict"`, passing `null` as the context object
to `.call` or `.apply` resulted in `this` taking on the value of the
global object inside the invoked function.

Technically the `"use strict"` directive is supposed to make it possible
that `this === null`, but strict mode is not respected by all browsers,
including (unfortunately) PhantomJS.

Since these `expect`-ations are just testing binding behavior, let's not
make them also test strict mode `this` handling.

src/core/__tests__/ReactCompositeComponent-test.js

@@ -207,9 +207,10 @@ describe('ReactCompositeComponent', function() {
     var autoBound = instance.methodAutoBound;
     var explicitlyNotBound = instance.methodExplicitlyNotBound;
 
-    expect(explicitlyBound.call(null)).toBe(instance);
-    expect(autoBound.call(null)).toBe(instance);
-    expect(explicitlyNotBound.call(null)).toBe(null);
+    var context = {};
+    expect(explicitlyBound.call(context)).toBe(instance);
+    expect(autoBound.call(context)).toBe(instance);
+    expect(explicitlyNotBound.call(context)).toBe(context);
 
     expect(explicitlyBound.call(instance)).toBe(instance);
     expect(autoBound.call(instance)).toBe(instance);