add createExpectedError method and fix xhr-impl

src/log.js

@@ -202,7 +202,7 @@ export class Log {
   error(tag, var_args) {
     const error = this.error_.apply(this, arguments);
     if (error) {
-      this.win.setTimeout(() => {throw error;});
+      this.win.setTimeout(() => {throw /** @type {!Error} */ (error);});
     }
   }
 
@@ -216,7 +216,7 @@ export class Log {
     const error = this.error_.apply(this, arguments);
     if (error) {
       error.expected = true;
-      this.win.setTimeout(() => {throw error;});
+      this.win.setTimeout(() => {throw /** @type {!Error} */ (error);});
     }
   }
 
@@ -231,6 +231,18 @@ export class Log {
     return error;
   }
 
+  /**
+   * Creates an error object with its expected property set to true.
+   * @param {...*} var_args
+   * @return {!Error}
+   */
+  createExpectedError(var_args) {
+    const error = createErrorVargs.apply(null, arguments);
+    this.prepareError_(error);
+    error.expected = true;
+    return error;
+  }
+
   /**
    * Throws an error if the first argument isn't trueish.
    *

src/service/viewer-impl.js

@@ -334,7 +334,7 @@ export class Viewer {
           } else {
             resolve(this.win.document.referrer);
             if (this.unconfirmedReferrerUrl_ != this.win.document.referrer) {
-              dev().error(TAG_, 'Untrusted viewer referrer override: ' +
+              dev().expectedError(TAG_, 'Untrusted viewer referrer override: ' +
                   this.unconfirmedReferrerUrl_ + ' at ' +
                   this.messagingOrigin_);
               this.unconfirmedReferrerUrl_ = this.win.document.referrer;

src/service/xhr-impl.js

@@ -353,7 +353,7 @@ export function fetchPolyfill(input, opt_init) {
       }
       if (xhr.status < 100 || xhr.status > 599) {
         xhr.onreadystatechange = null;
-        reject(new Error(`Unknown HTTP status ${xhr.status}`));
+        reject(user().createExpectedError(`Unknown HTTP status ${xhr.status}`));
         return;
       }
 
@@ -365,10 +365,10 @@ export function fetchPolyfill(input, opt_init) {
       }
     };
     xhr.onerror = () => {
-      reject(new Error('Network failure'));
+      reject(user().createExpectedError('Network failure'));
     };
     xhr.onabort = () => {
-      reject(new Error('Request aborted'));
+      reject(user().createExpectedError('Request aborted'));
     };
 
     if (init.method == 'POST') {
@@ -394,7 +394,7 @@ function createXhrRequest(method, url) {
     xhr = new XDomainRequest();
     xhr.open(method, url);
   } else {
-    throw new Error('CORS is not supported');
+    throw dev().createExpectedError('CORS is not supported');
   }
   return xhr;
 }

test/functional/test-log.js

@@ -406,8 +406,15 @@ describe('Logging', () => {
       expect(false).to.be.true;
     });
 
+    it('should create expected error from message', () => {
+      const error = log.createExpectedError('test');
+      expect(error).to.be.instanceof(Error);
+      expect(error.expected).to.be.true;
+    });
+
     it('should create suffixed errors from message', () => {
       const error = log.createError('test');
+      expect(error.expected).to.be.undefined;
       expect(error).to.be.instanceof(Error);
       expect(isUserErrorMessage(error.message)).to.be.true;
       expect(error.message).to.contain('test');