Allow optional chaining in JS library code

This change verifies that closure can successfully lower the optional
chaining away on older browsers.

This also requires bumping of chrome and safari minimum versions.. so
maybe its not worth it?

Author: sbc100

src/settings.js

@@ -1747,10 +1747,10 @@ var AUTO_NATIVE_LIBRARIES = true;
 // versions >= MIN_FIREFOX_VERSION
 // are desired to work. Pass -sMIN_FIREFOX_VERSION=majorVersion to drop support
 // for Firefox versions older than < majorVersion.
-// Firefox ESR 68 was released on July 9, 2019.
+// Firefox ESR 74 was released on March 10, 2020
 // MAX_INT (0x7FFFFFFF, or -1) specifies that target is not supported.
 // [link]
-var MIN_FIREFOX_VERSION = 68;
+var MIN_FIREFOX_VERSION = 74;
 
 // Specifies the oldest version of desktop Safari to target. Version is encoded
 // in MMmmVV, e.g. 70101 denotes Safari 7.1.1.
@@ -1789,10 +1789,10 @@ var MIN_EDGE_VERSION = 0x7FFFFFFF;
 
 // Specifies the oldest version of Chrome. E.g. pass -sMIN_CHROME_VERSION=58 to
 // drop support for Chrome 57 and older.
-// Chrome 75.0.3770 was released on 2019-06-04
+// Chrome 80 was released on February 4, 2020
 // MAX_INT (0x7FFFFFFF, or -1) specifies that target is not supported.
 // [link]
-var MIN_CHROME_VERSION = 75;
+var MIN_CHROME_VERSION = 80;
 
 // Specifies minimum node version to target for the generated code.  This is
 // distinct from the minimum version required run the emscripten compiler.

test/test_other.py

@@ -12621,6 +12621,10 @@ def test_es5_transpile(self, args):
           [key]: 42,
         };
         err('value: ' + obj2[key]);
+
+        // optional chaining
+        let result = Module.foo?.doSomething();
+        err('result: ' + result);
       }
     });
     ''')
@@ -12635,13 +12639,15 @@ def check_for_es6(filename, expect):
         self.assertContained(['() => 2', '()=>2'], js)
         self.assertContained('const ', js)
         self.assertContained('let ', js)
+        self.assertContained('foo?.doSomething()', js)
       else:
         self.verify_es5(filename)
         self.assertNotContained('foo(arg=', js)
         self.assertNotContained('() => 2', js)
         self.assertNotContained('()=>2', js)
         self.assertNotContained('const ', js)
         self.assertNotContained('let ', js)
+        self.assertNotContained('foo?.doSomething()', js)
 
     # Check that under normal circumstances none of these features get
     # removed / transpiled.

tools/feature_matrix.py

@@ -23,6 +23,7 @@ class Feature(IntEnum):
   THREADS = auto()
   GLOBALTHIS = auto()
   PROMISE_ANY = auto()
+  OPTIONAL_CHAINING = auto()
 
 
 default_features = {Feature.SIGN_EXT, Feature.MUTABLE_GLOBALS}
@@ -71,6 +72,12 @@ class Feature(IntEnum):
     'safari': 140000,
     'node': 150000,
   },
+  Feature.OPTIONAL_CHAINING: {
+    'chrome': 80,
+    'firefox': 74,
+    'safari': 130400,
+    'node': 140000,
+  },
 }