[NTI] Don't warn about the use of 'symbol' in typeof.

Fixes #2320 on github.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=149059410

src/com/google/javascript/jscomp/NewTypeInference.java

@@ -2457,6 +2457,7 @@ private void checkInvalidTypename(Node typeString) {
       case "undefined":
       case "function":
       case "object":
+      case "symbol":
       case "unknown": // IE-specific type name
         break;
       default:

test/com/google/javascript/jscomp/NewTypeInferenceTest.java

@@ -2912,6 +2912,14 @@ public void testTypeof() {
         "  } else if (typeof x.prop.prop2 === 'number') {",
         "  }",
         "}"));
+
+    typeCheck(LINE_JOINER.join(
+        "function f(x) {",
+        "  if (typeof x === 'symbol') {",
+        // We don't know what symbols are yet.
+        "    var /** null */ n = x;",
+        "  }",
+        "}"));
   }
 
   public void testAssignWithOp() {