Emit PrePack information as global into bundle (optionally)

Summary:
Emit PrePack info into the generated bundle. The intention is for this to include information about enabled prepack options. Currently it only includes a flag if lazy objects are enabled.

Its emitted as global which is somewhat intrusive so its guarded by a flag currently and defaults to false.

Example:
```
var __PREPACK__ = {lazyObjects: false}
```

Reviewed By: NTillmann

Differential Revision: D8321765

fbshipit-source-id: 03a41b2cef84ea44c02c71e233b407e3ddc1197d

src/options.js

@@ -92,6 +92,7 @@ export type SerializerOptions = {
   inlineExpressions?: boolean,
   trace?: boolean,
   heapGraphFormat?: "DotLanguage" | "VISJS",
+  prepackInfo?: boolean,
 };
 
 export type PartialEvaluatorOptions = {

src/prepack-cli.js

@@ -80,6 +80,7 @@ function run(
     --repro                  Create a zip file with all information needed to reproduce a Prepack run"
     --cpuprofile             Create a CPU profile file for the run that can be loaded into the Chrome JavaScript CPU Profile viewer",
     --debugDiagnosticSeverity      FatalError | RecoverableError | Warning | Information (default = FatalError). Diagnostic level at which debugger will stop
+    --prepackInfo        Emit a global variable with information about the corresponding PrePack run.
   `;
   let args = Array.from(process.argv);
   args.splice(0, 2);
@@ -120,6 +121,7 @@ function run(
     residual: false,
     profile: false,
     reactEnabled: false,
+    prepackInfo: false,
   };
 
   let reproArguments = [];
@@ -356,6 +358,7 @@ fi
     },
     flags
   );
+
   if (heapGraphFilePath !== undefined) resolvedOptions.heapGraphFormat = "DotLanguage";
   if (lazyObjectsRuntime !== undefined && (resolvedOptions.delayInitializations || resolvedOptions.inlineExpressions)) {
     console.error("lazy objects feature is incompatible with delayInitializations and inlineExpressions options");

src/prepack-options.js

@@ -65,6 +65,7 @@ export type PrepackOptions = {|
   debugOutFilePath?: string,
   abstractValueImpliesMax?: number,
   debuggerConfigArgs?: DebuggerConfigArguments,
+  prepackInfo?: boolean,
 |};
 
 export function getRealmOptions({
@@ -130,6 +131,7 @@ export function getSerializerOptions({
   inlineExpressions = false,
   initializeMoreModules = false,
   trace = false,
+  prepackInfo = false,
 }: PrepackOptions): SerializerOptions {
   let result: SerializerOptions = {
     delayInitializations,
@@ -144,6 +146,7 @@ export function getSerializerOptions({
     profile,
     inlineExpressions,
     trace,
+    prepackInfo,
   };
   if (lazyObjectsRuntime !== undefined) {
     result.lazyObjectsRuntime = lazyObjectsRuntime;

src/serializer/ResidualHeapSerializer.js

@@ -2236,6 +2236,23 @@ export class ResidualHeapSerializer {
       );
     }
 
+    if (this._options.prepackInfo) {
+      this.emitter.emit(
+        t.expressionStatement(
+          t.assignmentExpression(
+            "=",
+            this.preludeGenerator.globalReference("__PREPACK__", false),
+            t.objectExpression([
+              t.objectProperty(
+                t.identifier("lazyObjects"),
+                t.booleanLiteral(this._options.lazyObjectsRuntime !== undefined)
+              ),
+            ])
+          )
+        )
+      );
+    }
+
     this.postGeneratorSerialization();
 
     Array.prototype.push.apply(this.prelude, this.preludeGenerator.prelude);