Unit test failure; clearHostScreen function

Author: Josh Goldberg

src/compiler/watch.ts

@@ -302,9 +302,7 @@ namespace ts {
         // There is no extra check needed since we can just rely on the program to decide emit
         const builder = createBuilder({ getCanonicalFileName, computeHash });
 
-        if (watchingHost.system.clearScreen) {
-            watchingHost.system.clearScreen();
-        }
+        clearHostScreen();
         reportWatchDiagnostic(createCompilerDiagnostic(Diagnostics.Starting_compilation_in_watch_mode));
         synchronizeProgram();
 
@@ -496,10 +494,14 @@ namespace ts {
             scheduleProgramUpdate();
         }
 
-        function updateProgram() {
+        function clearHostScreen() {
             if (watchingHost.system.clearScreen) {
                 watchingHost.system.clearScreen();
             }
+        }
+
+        function updateProgram() {
+            clearHostScreen();
 
             timerToUpdateProgram = undefined;
             reportWatchDiagnostic(createCompilerDiagnostic(Diagnostics.File_change_detected_Starting_incremental_compilation));

src/harness/unittests/tscWatchMode.ts

@@ -117,6 +117,10 @@ namespace ts.tscWatch {
             index++;
         });
         if (!skipWaiting) {
+            if (errorsPosition === ExpectedOutputErrorsPosition.BeforeCompilationStarts) {
+                assertWatchDiagnosticAt(host, index, ts.Diagnostics.Starting_compilation_in_watch_mode);
+                index += 1;
+            }
             assertWatchDiagnosticAt(host, index, Diagnostics.Compilation_complete_Watching_for_file_changes);
         }
         host.clearOutput();