Reapply "Speed up TypeScript v2 (#6406)" (#6586)

This reverts commit b2cf28b7f1 and reapplies #6406.

packages/react-dev-utils/webpackHotDevClient.js

@@ -106,7 +106,7 @@ function handleSuccess() {
     tryApplyUpdates(function onHotUpdateSuccess() {
       // Only dismiss it when we're sure it's a hot update.
       // Otherwise it would flicker right before the reload.
-      ErrorOverlay.dismissBuildError();
+      tryDismissErrorOverlay();
     });
   }
 }
@@ -140,19 +140,15 @@ function handleWarnings(warnings) {
     }
   }
 
+  printWarnings();
+
   // Attempt to apply hot updates or reload.
   if (isHotUpdate) {
     tryApplyUpdates(function onSuccessfulHotUpdate() {
-      // Only print warnings if we aren't refreshing the page.
-      // Otherwise they'll disappear right away anyway.
-      printWarnings();
       // Only dismiss it when we're sure it's a hot update.
       // Otherwise it would flicker right before the reload.
-      ErrorOverlay.dismissBuildError();
+      tryDismissErrorOverlay();
     });
-  } else {
-    // Print initial warnings immediately.
-    printWarnings();
   }
 }
 
@@ -183,6 +179,12 @@ function handleErrors(errors) {
   // We will reload on next success instead.
 }
 
+function tryDismissErrorOverlay() {
+  if (!hasCompileErrors) {
+    ErrorOverlay.dismissBuildError();
+  }
+}
+
 // There is a newer version of the code available.
 function handleAvailableHash(hash) {
   // Update last known compilation hash.