Helper to inspect values at a particular AST node (#2554)

Summary:
This is a helper for inspecting values at a particular node in the AST. E.g.

```js

let n = global.__abstract ? __abstract("number", "10") : 10;
let x = {foo:1};
let y = {foo:2};
let c = __abstract("boolean", "c");

let i = 0;
let obj = {};
do {
  i++;
  obj.j = i;
  obj.foo = c ? x : y;
} while (i < n);

__debugValue(obj);        // Breaks with obj in context

inspect = function() {
  return i + " " + obj.j;
};
```
Pull Request resolved: https://github.com/facebook/prepack/pull/2554

Differential Revision: D9922908

Pulled By: sb98052

fbshipit-source-id: dab9cae64a461283d8dec7f0bb7f8fae87a01c78

src/intrinsics/prepack/global.js

@@ -398,6 +398,25 @@ export default function(realm: Realm): void {
     })
   );
 
+  // Helper function for Prepack developers inspect a value
+  // when interpreting a particular node in the AST.
+  global.$DefineOwnProperty(
+    "__debugValue",
+    new PropertyDescriptor({
+      value: createNativeFunctionForResidualInjection(
+        "__debugValue",
+        ([v, s]): void => {
+          debugger; // eslint-disable-line no-debugger
+        },
+        createOperationDescriptor("NOOP"),
+        2
+      ),
+      writable: true,
+      enumerable: false,
+      configurable: true,
+    })
+  );
+
   // Helper function that identifies a computation that must remain part of the residual program and cannot be partially evaluated,
   // e.g. because it contains a loop over abstract values.
   // __residual(typeNameOrTemplate, function, arg0, arg1, ...) creates a new abstract value

src/serializer/ResidualOperationSerializer.js

@@ -133,6 +133,9 @@ export class ResidualOperationSerializer {
       case "LOCAL_ASSIGNMENT":
         babelNode = this._serializeLocalAssignment(data, nodes, context, valuesToProcess);
         break;
+      case "NOOP":
+        babelNode = t.emptyStatement();
+        break;
       case "OBJECT_SET_PARTIAL":
         babelNode = this._serializeObjectSetPartial(data, nodes);
         break;

src/utils/generator.js

@@ -103,6 +103,7 @@ export type OperationDescriptorType =
   | "LOGICAL_PROPERTY_ASSIGNMENT"
   | "MODULES_REQUIRE"
   | "NEW_EXPRESSION"
+  | "NOOP"
   | "OBJECT_ASSIGN"
   | "OBJECT_GET_PARTIAL"
   | "OBJECT_PROTO_GET_OWN_PROPERTY_DESCRIPTOR"

test/serializer/abstract/DebugValue.js

@@ -0,0 +1,9 @@
+let x = global.__abstract ? __abstract("boolean", "true") : true;
+let ob = x ? { a: 1 } : { b: 2 };
+
+global.__abstract ? __debugValue(ob) : {};
+global.__abstract ? __debugValue([ob, x]) : {};
+
+inspect = function() {
+  return true;
+};