fix(apigateway): failing build on domain name with upercase

Api Gateway doesn't accept domain name with upercase letters as per [its documentation](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-apigateway-domainname.html#aws-resource-apigateway-domainname-properties).

This lead me to find out at CF deployment that my stack had an issue when this could have been discovered at build time.

fixes #8428

packages/@aws-cdk/aws-apigateway/lib/domain-name.ts

@@ -1,5 +1,5 @@
 import * as acm from '@aws-cdk/aws-certificatemanager';
-import { Construct, IResource, Resource } from '@aws-cdk/core';
+import { Construct, IResource, Resource, Token } from '@aws-cdk/core';
 import { CfnDomainName } from './apigateway.generated';
 import { BasePathMapping, BasePathMappingOptions } from './base-path-mapping';
 import { EndpointType, IRestApi } from './restapi';
@@ -102,6 +102,11 @@ export class DomainName extends Resource implements IDomainName {
     const endpointType = props.endpointType || EndpointType.REGIONAL;
     const edge = endpointType === EndpointType.EDGE;
 
+    if (!Token.isUnresolved(props.domainName) && /[A-Z]/.test(props.domainName)) {
+      throw new Error('domainName does not support uppercase letters. ' +
+        `got: '${props.domainName}'`);
+    }
+
     const resource = new CfnDomainName(this, 'Resource', {
       domainName: props.domainName,
       certificateArn: edge ? props.certificate.certificateArn : undefined,

packages/@aws-cdk/aws-apigateway/test/test.domains.ts

@@ -267,6 +267,21 @@ export = {
     test.done();
   },
 
+  'domain name cannot contain uppercase letters'(test: Test) {
+    // GIVEN
+    const stack = new Stack();
+    const certificate = new acm.Certificate(stack, 'cert', { domainName: 'someDomainWithUpercase.domain.com' });
+
+    // WHEN
+    test.throws(() => { 
+      new apigw.DomainName(stack, 'someDomain', {domainName: 'someDomainWithUpercase.domain.com', certificate}) 
+    })
+
+    // THEN
+
+    test.done();
+  },
+
   'multiple domain names can be added'(test: Test) {
     // GIVEN
     const domainName = 'my.domain.com';