feat(@embark/embarkjs): introduce Blockchain.isAvailable() API

This is in order to be consistent with existing `isAvailable()` APIs on
the `EmbarkJS` module.

Another reason this API is needed, is because we need a way to fire
`EmbarkJS.onReady()` even if no blockchain is available. Prior to this
commit, `EmbarkJS.onReady()` was purely relying on an API on `EmbarkJS.Blockchain`
that would break if the blockchain feature was disabled, causing
`EmbarkJS.onReady()` to never fire.

dapps/templates/demo/app/dapp.js

@@ -27,13 +27,16 @@ class App extends React.Component {
 
   componentDidMount() {
     EmbarkJS.onReady((err) => {
-      this.setState({blockchainEnabled: true});
       if (err) {
         // If err is not null then it means something went wrong connecting to ethereum
         // you can use this to ask the user to enable metamask for e.g
         return this.setState({error: err.message || err});
       }
 
+      EmbarkJS.Blockchain.isAvailable().then(result => {
+        this.setState({blockchainEnabled: result});
+      });
+
       EmbarkJS.Messages.isAvailable().then(result => {
         this.setState({whisperEnabled: result});
       });

packages/embarkjs/embarkjs/src/lib/blockchain.js

@@ -89,6 +89,13 @@ Blockchain.setProvider = function(providerName, options) {
   provider.init(options);
 };
 
+Blockchain.isAvailable = function () {
+  if (!this.blockchainConnector) {
+    return Promise.resolve(false);
+  }
+  return this.blockchainConnector.getNetworkId().then(_ => true);
+};
+
 Blockchain.doConnect = function(connectionList, opts, doneCb) {
   const self = this;
 

packages/embarkjs/embarkjs/src/lib/index.js

@@ -6,7 +6,13 @@ import Utils from './utils';
 
 var EmbarkJS = {
   onReady: function (cb) {
-    Blockchain.execWhenReady(cb);
+    Blockchain.isAvailable()
+      .then(available => {
+        if (available) {
+          return Blockchain.execWhenReady(cb);
+        }
+        cb();
+      });
   },
   enableEthereum: function () {
     return Blockchain.enableEthereum();