feat(compiler): read and write .ngsummary.json files

When compiling libraries, this feature extracts the minimal information
from the directives/pipes/modules of the library into `.ngsummary.json` files,
so that applications that use this library only need to be recompiled
if one of the summary files change, but not on every change
of the libraries (e.g. one of the templates).

Only works if individual codegen for libraries is enabled,
see the `generateCodeForLibraries: false` option.

Closes #12787

modules/@angular/compiler-cli/src/codegen.ts

@@ -22,6 +22,7 @@ import {PathMappedCompilerHost} from './path_mapped_compiler_host';
 import {Console} from './private_import_core';
 
 const GENERATED_FILES = /\.ngfactory\.ts$|\.css\.ts$|\.css\.shim\.ts$/;
+const GENERATED_META_FILES = /\.json$/;
 const GENERATED_OR_DTS_FILES = /\.d\.ts$|\.ngfactory\.ts$|\.css\.ts$|\.css\.shim\.ts$/;
 
 const PREAMBLE = `/**
@@ -68,10 +69,11 @@ export class CodeGenerator {
             sf => this.ngCompilerHost.getCanonicalFileName(sf.fileName)))
         .then(generatedModules => {
           generatedModules.forEach(generatedModule => {
-            const sourceFile = this.program.getSourceFile(generatedModule.fileUrl);
-            const emitPath = this.calculateEmitPath(generatedModule.moduleUrl);
-            this.host.writeFile(
-                emitPath, PREAMBLE + generatedModule.source, false, () => {}, [sourceFile]);
+            const sourceFile = this.program.getSourceFile(generatedModule.srcFileUrl);
+            const emitPath = this.calculateEmitPath(generatedModule.genFileUrl);
+            const source = GENERATED_META_FILES.test(emitPath) ? generatedModule.source :
+                                                                 PREAMBLE + generatedModule.source;
+            this.host.writeFile(emitPath, source, false, () => {}, [sourceFile]);
           });
         });
   }

modules/@angular/compiler-cli/src/compiler_host.ts

@@ -209,6 +209,12 @@ export class CompilerHost implements AotCompilerHost {
   }
 
   loadResource(filePath: string): Promise<string> { return this.context.readResource(filePath); }
+
+  loadSummary(filePath: string): string { return this.context.readFile(filePath); }
+
+  getOutputFileName(sourceFilePath: string): string {
+    return sourceFilePath.replace(EXT, '') + '.d.ts';
+  }
 }
 
 export class CompilerHostContextAdapter {