tests: remove a few invariants (#2383)

Author: IvanGoncharov

src/execution/__tests__/nonnull-test.js

@@ -3,8 +3,6 @@
 import { expect } from 'chai';
 import { describe, it } from 'mocha';
 
-import invariant from '../../jsutils/invariant';
-
 import { parse } from '../../language/parser';
 
 import { GraphQLSchema } from '../../type/schema';
@@ -567,10 +565,7 @@ describe('Execute: handles non-nullable types', () => {
                 type: GraphQLNonNull(GraphQLString),
               },
             },
-            resolve: (_, args) => {
-              invariant(typeof args.cannotBeNull === 'string');
-              return 'Passed: ' + args.cannotBeNull;
-            },
+            resolve: (_, args) => 'Passed: ' + String(args.cannotBeNull),
           },
         },
       }),

src/subscription/__tests__/subscribe-test.js

@@ -8,8 +8,6 @@ import EventEmitter from 'events';
 import { expect } from 'chai';
 import { describe, it } from 'mocha';
 
-import invariant from '../../jsutils/invariant';
-
 import { parse } from '../../language/parser';
 
 import { GraphQLError } from '../../error/GraphQLError';
@@ -144,7 +142,7 @@ async function expectPromiseToThrow(promise, message) {
     /* istanbul ignore next */
     expect.fail('promise should have thrown but did not');
   } catch (error) {
-    invariant(error instanceof Error);
+    expect(error).to.be.an.instanceOf(Error);
     expect(error.message).to.equal(message);
   }
 }