Use the correct CLI option to enable TDZ in Hermes

Fixes tc39#125

lib/agents/hermes.js

@@ -32,7 +32,23 @@ class HermesAgent extends ConsoleAgent {
       this.args.unshift('-non-strict');
     }
 
-    this.args.unshift('-Xintl', '-enable-eval', '-fenable-tdz');
+    // https://github.com/facebook/hermes/commit/f811a0e75f19ab0da7dcf969a61db894fbb3cc35
+    let tdzArg = '-Xenable-tdz'; 
+    try {
+      const cp = await this.createChildProcess(['-version']);
+      let stdout = '';
+      cp.stdout.on('data', str => { stdout += str; });
+      await new Promise(resolve => { cp.on('close', resolve); });
+      // https://github.com/facebook/hermes/blob/66d3f87a8fd0436634be8a1049a1db9d3aa6e9c0/lib/CompilerDriver/CompilerDriver.cpp#L2164
+      const [_, versionStr] = /Hermes release version: (.*)/.exec(stdout);
+      const [major, minor] = versionStr.split('.');
+      if (major === '0' && Number(minor) < 10) {
+        tdzArg = '-fenable-tdz';
+      }
+    } catch (error) {
+      // suppressed
+    }
+    this.args.unshift('-Xintl', '-enable-eval', tdzArg);
 
     // There is currently no flag for supporting modules in Hermes
     // if (options.module && this.args[0] !== '-m') {