fix(@ngtools/webpack): don't invalidate cache after first run

At the moment, since there are no old files in the compilation it will cause all source files to be invalidate after the first run. This shouldn't be done as it will slow down the 2nd recompilation.

packages/ngtools/webpack/src/angular_compiler_plugin.ts

@@ -400,12 +400,7 @@ export class AngularCompilerPlugin {
 
     // Use an identity function as all our paths are absolute already.
     this._moduleResolutionCache = ts.createModuleResolutionCache(this._basePath, x => x);
-
-    const tsProgram = this._getTsProgram();
-    const oldFiles = new Set(tsProgram ?
-      tsProgram.getSourceFiles().map(sf => sf.fileName)
-      : [],
-    );
+    const oldTsProgram = this._getTsProgram();
 
     if (this._JitMode) {
       // Create the TypeScript program.
@@ -414,14 +409,9 @@ export class AngularCompilerPlugin {
         this._rootNames,
         this._compilerOptions,
         this._compilerHost,
-        tsProgram,
+        oldTsProgram,
       );
       timeEnd('AngularCompilerPlugin._createOrUpdateProgram.ts.createProgram');
-
-      const newFiles = this._program.getSourceFiles().filter(sf => !oldFiles.has(sf.fileName));
-      for (const newFile of newFiles) {
-        this._compilerHost.invalidate(newFile.fileName);
-      }
     } else {
       time('AngularCompilerPlugin._createOrUpdateProgram.ng.createProgram');
       // Create the Angular program.
@@ -436,9 +426,14 @@ export class AngularCompilerPlugin {
       time('AngularCompilerPlugin._createOrUpdateProgram.ng.loadNgStructureAsync');
       await this._program.loadNgStructureAsync();
       timeEnd('AngularCompilerPlugin._createOrUpdateProgram.ng.loadNgStructureAsync');
+    }
 
-      const newFiles = this._program.getTsProgram()
-        .getSourceFiles().filter(sf => !oldFiles.has(sf.fileName));
+    const newTsProgram = this._getTsProgram();
+    if (oldTsProgram && newTsProgram) {
+      // The invalidation should only happen if we have an old program
+      // as otherwise we will invalidate all the sourcefiles.
+      const oldFiles = new Set(oldTsProgram.getSourceFiles().map(sf => sf.fileName));
+      const newFiles = newTsProgram.getSourceFiles().filter(sf => !oldFiles.has(sf.fileName));
       for (const newFile of newFiles) {
         this._compilerHost.invalidate(newFile.fileName);
       }
@@ -1056,7 +1051,8 @@ export class AngularCompilerPlugin {
         // We also need to all changed files as dependencies of this file, so that all of them
         // will be watched and trigger a rebuild next time.
         outputText = '';
-        errorDependencies = this._getChangedCompilationFiles()
+        const program = this._getTsProgram();
+        errorDependencies = (program ? program.getSourceFiles().map(x => x.fileName) : [])
           // These paths are used by the loader so we must denormalize them.
           .map((p) => this._compilerHost.denormalizePath(p));
       }