Issue 6090 - Allow use of console to be seen during tests

test/_common.js

Index: test/_common.js
===================================================================
--- a/test/_common.js
+++ b/test/_common.js
@@ -87,18 +87,18 @@
 
       throw new Error(
         "Attempt to import unknown JavaScript module " + resource
       );
     },
     reportError(e) {}
   },
   console: {
-    log() {},
-    error() {}
+    log: console.log.bind(console),
+    error: console.error.bind(console)
   },
   navigator: {
   },
   onShutdown: {
     add() {}
   },
   URL
 };
@@ -414,16 +414,37 @@
     Date: {
       now: () => currentTime
     }
   };
 };
 
 console.warn = console.log;
 
+exports.silenceAssertionOutput = function(test, msg)
+{
+  let msgMatch = new RegExp(`^Error: ${msg}[\r\n]`);
+  let errorHandler = globals.console.error;
+  globals.console.error = s =>
+  {
+    if (!msgMatch.test(s))
+      errorHandler(s);
+  };
+  let result;
+  try
+  {
+    result = test();

[sergei, 2017/11/24 16:46:27]

There is no necessity in an additional variable, could you please just return
from the try-block?

[hub, 2017/11/24 18:17:56]

For some reason I believed that return would by-pass the finally.... 

Done

+  }
+  finally
+  {
+    globals.console.error = errorHandler;
+  }
+  return result;
+};
+
 exports.setupRandomResult = function()
 {
   let randomResult = 0.5;
   Object.defineProperty(this, "randomResult", {
     get: () => randomResult,
     set: value => randomResult = value
   });