Make injected web3 fail hard on sync methods (#471)

Make injected web3 fail hard on sync methods

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Current Master
 
+- MetaMask now throws descriptive errors when apps try to use synchronous web3 methods.
+
 ## 2.6.2 2016-07-20
 
 - Fixed bug that would prevent the plugin from reopening on the first try after receiving a new transaction while locked.

app/scripts/lib/inpage-provider.js

@@ -107,7 +107,15 @@ function createSyncProvider (providerConfig) {
         syncProviderUrl = MetamaskConfig.network.default
     }
   }
-  return new HttpProvider(syncProviderUrl)
+
+  const provider = new HttpProvider(syncProviderUrl)
+  // Stubbing out the send method to throw on sync methods:
+  provider.send = function() {
+    var message = 'The MetaMask Web3 object does not support synchronous methods. See https://github.com/MetaMask/faq#all-async---think-of-metamask-as-a-light-client for details.'
+    throw new Error(message)
+  }
+
+  return provider
 }
 
 function remoteStoreWithLocalStorageCache (storageKey) {