Change erc-20 package compiler target to istanbul

contracts/erc20/contracts/src/WETH9.sol

@@ -14,7 +14,7 @@
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 // solhint-disable
-pragma solidity ^0.4.18;
+pragma solidity ^0.5.9;
 
 
 contract WETH9 {
@@ -30,27 +30,27 @@ contract WETH9 {
     mapping (address => uint)                       public  balanceOf;
     mapping (address => mapping (address => uint))  public  allowance;
 
-    function() public payable {
+    function() external payable {
         deposit();
     }
     function deposit() public payable {
         balanceOf[msg.sender] += msg.value;
-        Deposit(msg.sender, msg.value);
+        emit Deposit(msg.sender, msg.value);
     }
     function withdraw(uint wad) public {
-        require(balanceOf[msg.sender] >= wad);
+        require(balanceOf[msg.sender] >= wad, "WETH9/INSUFFICIENT_BALANCE");
         balanceOf[msg.sender] -= wad;
         msg.sender.transfer(wad);
-        Withdrawal(msg.sender, wad);
+        emit Withdrawal(msg.sender, wad);
     }
 
     function totalSupply() public view returns (uint) {
-        return this.balance;
+        return address(this).balance;
     }
 
     function approve(address guy, uint wad) public returns (bool) {
         allowance[msg.sender][guy] = wad;
-        Approval(msg.sender, guy, wad);
+        emit Approval(msg.sender, guy, wad);
         return true;
     }
 
@@ -62,17 +62,17 @@ contract WETH9 {
         public
         returns (bool)
     {
-        require(balanceOf[src] >= wad);
+        require(balanceOf[src] >= wad), "WETH9/INSUFFICIENT_BALANCE";
 
         if (src != msg.sender && allowance[src][msg.sender] != uint(-1)) {
-            require(allowance[src][msg.sender] >= wad);
+            require(allowance[src][msg.sender] >= wad, "WETH9/INSUFFICIENT_ALLOWANCE");
             allowance[src][msg.sender] -= wad;
         }
 
         balanceOf[src] -= wad;
         balanceOf[dst] += wad;
 
-        Transfer(src, dst, wad);
+        emit Transfer(src, dst, wad);
 
         return true;
     }