perf(ivy): switch ngtsc to use single-file emit

In the TypeScript compiler API, emit() can be performed either on a single
ts.SourceFile or on the entire ts.Program simultaneously.

ngtsc previously used whole-program emit, which was convenient to use while
spinning up the project but has a significant drawback: it causes a type
checking operation to occur for the whole program, including .d.ts files.
In large Bazel environments (such as Google's codebase), an ngtsc invocation
can have a few .ts files and thousands of .d.ts inputs. This unwanted type
checking is therefore a significant drain on performance.

This commit switches ngtsc to emit each .ts file individually, avoiding the
unwanted type checking.

packages/compiler-cli/src/ngtsc/program.ts

@@ -57,6 +57,8 @@ export class NgtscProgram implements api.Program {
   private refEmitter: ReferenceEmitter|null = null;
   private fileToModuleHost: FileToModuleHost|null = null;
 
+  private generatedToOriginalSourceFiles = new Map<ts.SourceFile, ts.SourceFile>();
+
   constructor(
       rootNames: ReadonlyArray<string>, private options: api.CompilerOptions,
       host: api.CompilerHost, oldProgram?: api.Program) {
@@ -269,6 +271,8 @@ export class NgtscProgram implements api.Program {
           if (this.closureCompilerEnabled && fileName.endsWith('.js')) {
             data = nocollapseHack(data);
           }
+          let outputSourceFiles = sourceFiles;
+
           this.host.writeFile(fileName, data, writeByteOrderMark, onError, sourceFiles);
         };
 
@@ -293,19 +297,28 @@ export class NgtscProgram implements api.Program {
       beforeTransforms.push(...customTransforms.beforeTs);
     }
 
+    const emitResults: ts.EmitResult[] = [];
+    for (const targetSourceFile of this.tsProgram.getSourceFiles()) {
+      if (targetSourceFile.isDeclarationFile) {
+        continue;
+      }
+
+      emitResults.push(emitCallback({
+        targetSourceFile,
+        program: this.tsProgram,
+        host: this.host,
+        options: this.options,
+        emitOnlyDtsFiles: false, writeFile,
+        customTransformers: {
+          before: beforeTransforms,
+          after: customTransforms && customTransforms.afterTs,
+          afterDeclarations: afterDeclarationsTransforms,
+        },
+      }));
+    }
+
     // Run the emit, including a custom transformer that will downlevel the Ivy decorators in code.
-    const emitResult = emitCallback({
-      program: this.tsProgram,
-      host: this.host,
-      options: this.options,
-      emitOnlyDtsFiles: false, writeFile,
-      customTransformers: {
-        before: beforeTransforms,
-        after: customTransforms && customTransforms.afterTs,
-        afterDeclarations: afterDeclarationsTransforms,
-      },
-    });
-    return emitResult;
+    return ((opts && opts.mergeEmitResultsCallback) || mergeEmitResults)(emitResults);
   }
 
   private compileTypeCheckProgram(ctx: TypeCheckContext): ReadonlyArray<ts.Diagnostic> {