fix(@ngtools/webpack): don't show loader errors after compilation fails (angular#7575)

Author: filipesilva

packages/@ngtools/webpack/src/loader.ts

@@ -602,7 +602,14 @@ export function ngcLoader(this: LoaderContext & { _compilation: any }, source: s
         });
 
         const result = refactor.transpile(compilerOptions);
-        cb(null, result.outputText, result.sourceMap);
+
+        if (plugin.failedCompilation) {
+          // Return an empty string to prevent extra loader errors (missing imports etc).
+          // Plugin errors were already pushed to the compilation errors.
+          cb(null, '');
+        } else {
+          cb(null, result.outputText, result.sourceMap);
+        }
       })
       .catch(err => cb(err));
   } else {

packages/@ngtools/webpack/src/plugin.ts

@@ -61,6 +61,7 @@ export class AotPlugin implements Tapable {
   private _donePromise: Promise<void> | null;
   private _compiler: any = null;
   private _compilation: any = null;
+  private _failedCompilation = false;
 
   private _typeCheck = true;
   private _skipCodeGeneration = false;
@@ -87,6 +88,7 @@ export class AotPlugin implements Tapable {
   get compilerHost() { return this._compilerHost; }
   get compilerOptions() { return this._compilerOptions; }
   get done() { return this._donePromise; }
+  get failedCompilation() { return this._failedCompilation; }
   get entryModule() {
     const splitted = this._entryModule.split('#');
     const path = splitted[0];
@@ -374,11 +376,14 @@ export class AotPlugin implements Tapable {
 
     compiler.plugin('make', (compilation: any, cb: any) => this._make(compilation, cb));
     compiler.plugin('after-emit', (compilation: any, cb: any) => {
-      this._donePromise = null;
-      this._compilation = null;
       compilation._ngToolsWebpackPluginInstance = null;
       cb();
     });
+    compiler.plugin('done', () => {
+      this._donePromise = null;
+      this._compilation = null;
+      this._failedCompilation = false;
+    });
 
     compiler.plugin('after-resolvers', (compiler: any) => {
       // Virtual file system.
@@ -578,10 +583,13 @@ export class AotPlugin implements Tapable {
       .then(() => {
         if (this._compilation.errors == 0) {
           this._compilerHost.resetChangedFileTracker();
+        } else {
+          this._failedCompilation = true;
         }
 
         cb();
       }, (err: any) => {
+        this._failedCompilation = true;
         compilation.errors.push(err.stack);
         cb();
       });