[update] Test coverage

tests/invariant-test.js

@@ -0,0 +1,17 @@
+describe('invariant', function() {
+  let invariant = require('../invariant')
+
+  it ('throws an error if a condition is false', function(done) {
+    try {
+      invariant(false, 'message')
+    } catch(x) {
+      x.should.be.instanceof(Error)
+      done()
+    }
+  })
+
+  it ('does not throw an error if the condition is true', function() {
+    invariant(true, 'message')
+  })
+
+})