Do not create program multiple times when there are no changes in files (#747)

TypeScript cant tell if there is invalidation because of custom module resolution, that means it needs to create new program whenver asked for
Fixes #746

Author: sheetalkamat

src/index.ts

@@ -255,6 +255,7 @@ function updateFileInCache(
   }
 
   if (instance.watchHost && fileWatcherEventKind !== undefined) {
+    instance.hasUnaccountedModifiedFiles = true;
     instance.watchHost.invokeFileWatcher(filePath, fileWatcherEventKind);
     instance.watchHost.invokeDirectoryWatcher(path.dirname(filePath), filePath);
   }

src/instances.ts

@@ -25,13 +25,16 @@ const instances = <TSInstances>{};
 
 function ensureProgram(instance: TSInstance) {
   if (instance && instance.watchHost) {
-    if (instance.changedFilesList) {
-      instance.watchHost.updateRootFileNames();
-    }
-    if (instance.watchOfFilesAndCompilerOptions) {
-      instance.program = instance.watchOfFilesAndCompilerOptions
-        .getProgram()
-        .getProgram();
+    if (instance.hasUnaccountedModifiedFiles) {
+      if (instance.changedFilesList) {
+        instance.watchHost.updateRootFileNames();
+      }
+      if (instance.watchOfFilesAndCompilerOptions) {
+        instance.program = instance.watchOfFilesAndCompilerOptions
+          .getProgram()
+          .getProgram();
+      }
+      instance.hasUnaccountedModifiedFiles = false;
     }
     return instance.program;
   }

src/interfaces.ts

@@ -256,6 +256,7 @@ export interface TSInstance {
     typescript.BuilderProgram
   >;
   program?: typescript.Program;
+  hasUnaccountedModifiedFiles?: boolean;
   changedFilesList?: boolean;
 }