fix(@ngtools/webpack): avoid re-running full emit when there wasn't a an emit

There can be a number of reason why `_emitSkipped` is set to true these are:
- Errors have been encountered
- Changes are outside of TS compilations such as HTML and CSS in JIT mode

We only want to run a full JIT emit when;
- There hasn't been a success emit
- When a large number of files have changed
- First run

If a subsequent JIT build fails we shouldn't do a full emit either as we will transpile only the files that changes after the success build

https://github.com/angular/angular-cli/blob/64243919c10c33775be71010efd5ae69622a728f/packages/ngtools/webpack/src/angular_compiler_plugin.ts#L1029-L1031

Fixes #14775

Author: alan-agius4

packages/ngtools/webpack/src/angular_compiler_plugin.ts

@@ -106,6 +106,9 @@ export class AngularCompilerPlugin {
   private _platform: PLATFORM;
   private _JitMode = false;
   private _emitSkipped = true;
+  // This is needed because if the first build fails we need to do a full emit
+  // even whe only a single file gets updated.
+  private _hadFullJitEmit: boolean | undefined;
   private _changedFileExtensions = new Set(['ts', 'tsx', 'html', 'css', 'js', 'json']);
 
   // Webpack plugin.
@@ -1191,14 +1194,15 @@ export class AngularCompilerPlugin {
           'AngularCompilerPlugin._emit.ts', diagMode));
 
         if (!hasErrors(allDiagnostics)) {
-          if (this._firstRun || changedTsFiles.size > 20 || this._emitSkipped) {
+          if (this._firstRun || changedTsFiles.size > 20 || !this._hadFullJitEmit) {
             emitResult = tsProgram.emit(
               undefined,
               undefined,
               undefined,
               undefined,
               { before: this._transformers },
             );
+            this._hadFullJitEmit = !emitResult.emitSkipped;
             allDiagnostics.push(...emitResult.diagnostics);
           } else {
             for (const changedFile of changedTsFiles) {