Add fake gas estimate suprovider for tests

packages/0x.js/src/contract.ts

@@ -58,7 +58,7 @@ export class Contract implements Web3.ContractInstance {
             const promise = new Promise(async (resolve, reject) => {
                 const lastArg = args[args.length - 1];
                 let txData: Partial<Web3.TxData> = {};
-                if (this.isTxData(lastArg)) {
+                if (!_.isUndefined(lastArg) && this.isTxData(lastArg)) {
                     txData = args.pop();
                 }
                 // Gas amount sourced with the following priorities:

packages/0x.js/test/utils/constants.ts

@@ -8,4 +8,5 @@ export const constants = {
     KOVAN_RPC_URL: 'https://kovan.infura.io',
     ROPSTEN_RPC_URL: 'https://ropsten.infura.io',
     ZRX_DECIMALS: 18,
+    GAS_ESTIMATE: 500000,
 };

packages/0x.js/test/utils/subproviders/empty_wallet_subprovider.ts

@@ -1,4 +1,4 @@
-import {JSONRPCPayload} from '../types';
+import {JSONRPCPayload} from '../../../src/types';
 
 /*
  * This class implements the web3-provider-engine subprovider interface and returns

packages/0x.js/test/utils/subproviders/fake_gas_estimate_subprovider.ts

@@ -0,0 +1,31 @@
+import {JSONRPCPayload} from '../../../src/types';
+
+/*
+ * This class implements the web3-provider-engine subprovider interface and returns
+ * the constant gas estimate when queried.
+ * Source: https://github.com/MetaMask/provider-engine/blob/master/subproviders/subprovider.js
+ */
+export class FakeGasEstimateProvider {
+    private constantGasAmount: number;
+    constructor(constantGasAmount: number) {
+        this.constantGasAmount = constantGasAmount;
+    }
+    // This method needs to be here to satisfy the interface but linter wants it to be static.
+    // tslint:disable-next-line:prefer-function-over-method
+    public handleRequest(payload: JSONRPCPayload, next: () => void, end: (err: Error|null, result: any) => void) {
+        switch (payload.method) {
+            case 'eth_estimateGas':
+                end(null, this.constantGasAmount);
+                return;
+
+            default:
+                next();
+                return;
+        }
+    }
+    // Required to implement this method despite not needing it for this subprovider
+    // tslint:disable-next-line:prefer-function-over-method
+    public setEngine(engine: any) {
+        // noop
+    }
+}

packages/0x.js/test/utils/web3_factory.ts

@@ -7,7 +7,8 @@ import * as Web3 from 'web3';
 import ProviderEngine = require('web3-provider-engine');
 import RpcSubprovider = require('web3-provider-engine/subproviders/rpc');
 
-import {EmptyWalletSubProvider} from '../../src/subproviders/empty_wallet_subprovider';
+import {EmptyWalletSubProvider} from './subproviders/empty_wallet_subprovider';
+import {FakeGasEstimateProvider} from './subproviders/fake_gas_estimate_subprovider';
 
 import {constants} from './constants';
 
@@ -24,6 +25,7 @@ export const web3Factory = {
         if (!hasAddresses) {
             provider.addProvider(new EmptyWalletSubProvider());
         }
+        provider.addProvider(new FakeGasEstimateProvider(constants.GAS_ESTIMATE));
         provider.addProvider(new RpcSubprovider({
             rpcUrl,
         }));