perf(compiler): only emit changed files for incremental compilation

For now, we always create all generated files, but diff them
before we pass them to TypeScript.

For the user files, we compare the programs and only emit changed
TypeScript files.

This also adds more diagnostic messages if the `—diagnostics` flag
is passed to the command line.

packages/compiler-cli/test/transformers/program_spec.ts

@@ -57,17 +57,20 @@ describe('ng program', () => {
   }
 
   function compile(
-      oldProgram?: ng.Program, overrideOptions?: ng.CompilerOptions,
-      rootNames?: string[]): ng.Program {
+      oldProgram?: ng.Program, overrideOptions?: ng.CompilerOptions, rootNames?: string[],
+      host?: CompilerHost): ng.Program {
     const options = testSupport.createCompilerOptions(overrideOptions);
     if (!rootNames) {
       rootNames = [path.resolve(testSupport.basePath, 'src/index.ts')];
     }
-
+    if (!host) {
+      host = ng.createCompilerHost({options});
+    }
     const program = ng.createProgram({
       rootNames: rootNames,
       options,
-      host: ng.createCompilerHost({options}), oldProgram,
+      host,
+      oldProgram,
     });
     expectNoDiagnosticsInProgram(options, program);
     program.emit();
@@ -153,6 +156,59 @@ describe('ng program', () => {
              .toBe(false);
        });
 
+    it('should only emit changed files', () => {
+      testSupport.writeFiles({
+        'src/index.ts': createModuleAndCompSource('comp', 'index.html'),
+        'src/index.html': `Start`
+      });
+      const options: ng.CompilerOptions = {declaration: false};
+      const host = ng.createCompilerHost({options});
+      const originalGetSourceFile = host.getSourceFile;
+      const fileCache = new Map<string, ts.SourceFile>();
+      host.getSourceFile = (fileName: string) => {
+        if (fileCache.has(fileName)) {
+          return fileCache.get(fileName);
+        }
+        const sf = originalGetSourceFile.call(host, fileName);
+        fileCache.set(fileName, sf);
+        return sf;
+      };
+
+      const written = new Map<string, string>();
+      host.writeFile = (fileName: string, data: string) => written.set(fileName, data);
+
+      // compile libraries
+      const p1 = compile(undefined, options, undefined, host);
+
+      // first compile without libraries
+      const p2 = compile(p1, options, undefined, host);
+      expect(written.has(path.resolve(testSupport.basePath, 'built/src/index.js'))).toBe(true);
+      let ngFactoryContent =
+          written.get(path.resolve(testSupport.basePath, 'built/src/index.ngfactory.js'));
+      expect(ngFactoryContent).toMatch(/Start/);
+
+      // no change -> no emit
+      written.clear();
+      const p3 = compile(p2, options, undefined, host);
+      expect(written.size).toBe(0);
+
+      // change a user file
+      written.clear();
+      fileCache.delete(path.resolve(testSupport.basePath, 'src/index.ts'));
+      const p4 = compile(p3, options, undefined, host);
+      expect(written.size).toBe(1);
+      expect(written.has(path.resolve(testSupport.basePath, 'built/src/index.js'))).toBe(true);
+
+      // change a file that is input to generated files
+      written.clear();
+      testSupport.writeFiles({'src/index.html': 'Hello'});
+      const p5 = compile(p4, options, undefined, host);
+      expect(written.size).toBe(1);
+      ngFactoryContent =
+          written.get(path.resolve(testSupport.basePath, 'built/src/index.ngfactory.js'));
+      expect(ngFactoryContent).toMatch(/Hello/);
+    });
+
     it('should store library summaries on emit', () => {
       compileLib('lib');
       testSupport.writeFiles({
@@ -163,17 +219,19 @@ describe('ng program', () => {
           `
       });
       const p1 = compile();
-      expect(p1.getLibrarySummaries().some(
-                 sf => /node_modules\/lib\/index\.ngfactory\.d\.ts$/.test(sf.fileName)))
+      expect(Array.from(p1.getLibrarySummaries().values())
+                 .some(sf => /node_modules\/lib\/index\.ngfactory\.d\.ts$/.test(sf.fileName)))
           .toBe(true);
-      expect(p1.getLibrarySummaries().some(
-                 sf => /node_modules\/lib\/index\.ngsummary\.json$/.test(sf.fileName)))
+      expect(Array.from(p1.getLibrarySummaries().values())
+                 .some(sf => /node_modules\/lib\/index\.ngsummary\.json$/.test(sf.fileName)))
           .toBe(true);
-      expect(
-          p1.getLibrarySummaries().some(sf => /node_modules\/lib\/index\.d\.ts$/.test(sf.fileName)))
+      expect(Array.from(p1.getLibrarySummaries().values())
+                 .some(sf => /node_modules\/lib\/index\.d\.ts$/.test(sf.fileName)))
           .toBe(true);
 
-      expect(p1.getLibrarySummaries().some(sf => /src\/main.*$/.test(sf.fileName))).toBe(false);
+      expect(Array.from(p1.getLibrarySummaries().values())
+                 .some(sf => /src\/main.*$/.test(sf.fileName)))
+          .toBe(false);
     });
 
     it('should reuse the old ts program completely if nothing changed', () => {