fix toString for some web objects (#2040)

js/custom_event.ts

@@ -52,6 +52,10 @@ export class CustomEvent extends event.Event implements domTypes.CustomEvent {
 
     customEventAttributes.set(this, { detail });
   }
+
+  get [Symbol.toStringTag](): string {
+    return "CustomEvent";
+  }
 }
 
 /** Built-in objects providing `get` methods for our

js/custom_event_test.ts

@@ -19,3 +19,9 @@ test(function customEventInitializedWithDetail() {
   assertEquals(event.target, null);
   assertEquals(event.type, type);
 });
+
+test(function toStringShouldBeWebCompatibility() {
+  const type = "touchstart";
+  const event = new CustomEvent(type, {});
+  assertEquals(event.toString(), "[object CustomEvent]");
+});

js/event_target.ts

@@ -53,4 +53,8 @@ export class EventTarget implements domTypes.EventTarget {
     }
     return !event.defaultPrevented;
   }
+
+  get [Symbol.toStringTag](): string {
+    return "EventTarget";
+  }
 }

js/event_target_test.ts

@@ -87,3 +87,8 @@ test(function constructedEventTargetUseObjectPrototype() {
   target.dispatchEvent(event);
   assertEquals(callCount, 2);
 });
+
+test(function toStringShouldBeWebCompatibility() {
+  const target = new EventTarget();
+  assertEquals(target.toString(), "[object EventTarget]");
+});

js/form_data.ts

@@ -136,6 +136,10 @@ class FormDataBase {
       }
     }
   }
+
+  get [Symbol.toStringTag](): string {
+    return "FormData";
+  }
 }
 
 export class FormData extends DomIterableMixin<

js/form_data_test.ts

@@ -166,3 +166,8 @@ test(function formDataParamsArgumentsCheck() {
     );
   });
 });
+
+test(function toStringShouldBeWebCompatibility() {
+  const formData = new FormData();
+  assertEquals(formData.toString(), "[object FormData]");
+});

js/headers.ts

@@ -125,6 +125,10 @@ class HeadersBase {
     this._validateValue(newvalue);
     this[headerMap].set(newname, newvalue);
   }
+
+  get [Symbol.toStringTag](): string {
+    return "Headers";
+  }
 }
 
 // @internal

js/headers_test.ts

@@ -316,3 +316,8 @@ test(function headerParamsArgumentsCheck() {
     );
   });
 });
+
+test(function toStringShouldBeWebCompatibility() {
+  const headers = new Headers();
+  assertEquals(headers.toString(), "[object Headers]");
+});