Adding multiple CustomButtons adds only the last CustomButton

[fiedler-s]

Bug Report or Feature Request (mark with an x)

- [ x] bug report -> please search issues before submitting
- [ ] feature request
- [ ] question

OS and Version?

All

Versions

Angular CLI: 1.7.0
Node: 9.1.0
OS: darwin x64
Angular: 5.2.1
... animations, common, compiler, compiler-cli, core, forms
... http, platform-browser, platform-browser-dynamic
... platform-server, router

@angular/cdk: 2.0.0-beta.12
@angular/cli: 1.7.0
@angular/material: 2.0.0-beta.12
@angular/service-worker: 5.1.3
@angular-devkit/build-optimizer: 0.3.1
@angular-devkit/core: 0.3.1
@angular-devkit/schematics: 0.3.1
@ngtools/json-schema: 1.2.0
@ngtools/webpack: 1.10.0
@schematics/angular: 0.3.1
@schematics/package-update: 0.3.1
typescript: 2.5.3
webpack-bundle-analyzer: 2.9.2
webpack-node-externals: 1.6.0
webpack: 3.11.0

Repro steps

constructor(private share: ShareButtons) {

    this.share.registerButton('xing', {
        type: 'xing',
        text: 'Xing',
        icon: 'fa fa-xing',
        share: {
            desktop: 'sharerURLForXing',  // the sharer link used to share on xing
            android: 'sharerURLForXing',  // the sharer link used to share on xing
            ios: 'sharerURLForXing',      // the sharer link used to share on xing
            operators: [noneOperator]
        }
    });

    this.share.registerButton('messenger', {
        type: 'messenger',
        text: 'Messenger',
        icon: 'fa fa-messenger',
        share: {
            desktop: 'sharerURLForMessenger',  // the sharer link used to share on Messenger
            android: 'sharerURLForMessenger',  // the sharer link used to share on Messenger
            ios: 'sharerURLForMessenger',      // the sharer link used to share on Messenger
            operators: [noneOperator]
        }
    });
}

The log given by the failure

Desired functionality

When adding multiple CustomButtons all will be added, not only the last one.

Mention any other details that might be useful

Error is in Method registerButton, it adds the CustomButton to the default ShareButtons and makes them the current ShareButtons.
Instead of adding the CustomButton to the default ShareButtons it should be added to the current set of ShareButtons, this way all the added CustomButtons will be present.