Printing with [@@toStringTag]

Summary:
For any built-in objects whose printing is not specialized, we can
prefix its 19.4.2.15 Symbol.toStringTag value to help with
disambiguating those objects.

This covers cases including Generator, {String, RegExp, Map, Set,
Array}Iterator, Math, JSON, etc.  They were all printed as an
meaningless `{  }` before.

Reviewed By: avp

Differential Revision: D22631553

fbshipit-source-id: b4536965ff97672e855e02c6afbb9e83054d55ed

test/repl/pretty.js

@@ -137,3 +137,22 @@ a = new Float64Array([]);
 // CHECK-NEXT: Float64Array [  ]
 a = new ArrayBuffer();
 // CHECK-NEXT: ArrayBuffer {  }
+
+g = (function*(){})();
+// CHECK-NEXT: Generator {  }
+
+it = new Map().entries()
+// CHECK-NEXT: Map Iterator {  }
+it = new Set().entries()
+// CHECK-NEXT: Set Iterator {  }
+it =  [][Symbol.iterator]();
+// CHECK-NEXT: Array Iterator {  }
+it = 'abc'[Symbol.iterator]();
+// CHECK-NEXT: String Iterator {  }
+it = 'abc'.matchAll(/1/g)
+// CHECK-NEXT: RegExp String Iterator {  }
+
+Math
+// CHECK-NEXT: Math {{{.*}}}
+JSON
+// CHECK-NEXT: JSON {{{.*}}}

tools/hermes/evaluate-line.js

@@ -190,10 +190,14 @@ C_STRING((function() {
     }
     if (value.constructor && value.constructor.name && value.constructor.name !== "Object") {
       return value.constructor.name + ' { ' + elements.join(', ') + ' }';
+    } else if (value[Symbol.toStringTag]) {
+      // For any built-in objects whose printing is not specialized, we prefix its
+      // 19.4.2.15 Symbol.toStringTag value to help with disambiguation. This covers cases
+      // including Generator, {String, RegExp, Map, Set, Array}Iterator, Math, JSON, etc.
+      return value[Symbol.toStringTag] + ' { ' + elements.join(', ') + ' }';
     } else {
       return '{ ' + elements.join(', ') + ' }';
     }
-
   }
 
   function prettyPrint(value, isColored) {