Merge pull request #4351 from ethereum-optimism/sc/ctb-legacy-eth-invariant

feat(ctb): preserve legacy ETH invariant

packages/contracts-bedrock/contracts/legacy/LegacyERC20ETH.sol

@@ -22,6 +22,19 @@ contract LegacyERC20ETH is OptimismMintableERC20 {
         OptimismMintableERC20(Predeploys.L2_STANDARD_BRIDGE, address(0), "Ether", "ETH")
     {}
 
+    /**
+     * @notice Returns the ETH balance of the target account. Overrides the base behavior of the
+     *         contract to preserve the invariant that the balance within this contract always
+     *         matches the balance in the state trie.
+     *
+     * @param _who Address of the account to query.
+     *
+     * @return The ETH balance of the target account.
+     */
+    function balanceOf(address _who) public view virtual override returns (uint256) {
+        return address(_who).balance;
+    }
+
     /**
      * @custom:blocked
      * @notice Mints some amount of ETH.