misc/wasm: use single map for string, symbol and object id mapping.

Currently we use a globally unique symbol property on objects that get
passed from JavaScript to Go to store a unique ID that Go then uses when
referring back to the JavaScript object (via js.Value.ref). This
approach fails however when a JavaScript object cannot be modified, i.e.
cannot have new properties added or is frozen. The test that is added as
part of this commit currently fails with:

  Cannot add property Symbol(), object is not extensible

Instead we consolidate the string, symbol and object unique ID mapping
into a single map. Map key equality is determined via strict equality,
which is the semantic we want in this situation.

Change-Id: Ieb2b50fc36d3c30e148aa7a41557f3c59cd33766
Reviewed-on: https://go-review.googlesource.com/121799
Run-TryBot: Paul Jolly <paul@myitcv.org.uk>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Richard Musiol <neelance@gmail.com>

misc/wasm/wasm_exec.js

@@ -118,33 +118,11 @@
 						return;
 				}
 
-				if (typeof v === "string") {
-					let ref = this._stringRefs.get(v);
-					if (ref === undefined) {
-						ref = this._values.length;
-						this._values.push(v);
-						this._stringRefs.set(v, ref);
-					}
-					mem().setUint32(addr, ref, true);
-					return;
-				}
-
-				if (typeof v === "symbol") {
-					let ref = this._symbolRefs.get(v);
-					if (ref === undefined) {
-						ref = this._values.length;
-						this._values.push(v);
-						this._symbolRefs.set(v, ref);
-					}
-					mem().setUint32(addr, ref, true);
-					return;
-				}
-
-				let ref = v[this._refProp];
-				if (ref === undefined || this._values[ref] !== v) {
+				let ref = this._refs.get(v);
+				if (ref === undefined) {
 					ref = this._values.length;
 					this._values.push(v);
-					v[this._refProp] = ref;
+					this._refs.set(v, ref);
 				}
 				mem().setUint32(addr, ref, true);
 			}
@@ -335,9 +313,7 @@
 					setTimeout(this._resolveCallbackPromise, 0); // make sure it is asynchronous
 				},
 			];
-			this._stringRefs = new Map();
-			this._symbolRefs = new Map();
-			this._refProp = Symbol();
+			this._refs = new Map();
 			this.exited = false;
 
 			const mem = new DataView(this._inst.exports.mem.buffer)

src/syscall/js/js_test.go

@@ -116,6 +116,14 @@ func TestObject(t *testing.T) {
 	}
 }
 
+func TestFrozenObject(t *testing.T) {
+	o := js.Global().Call("eval", "(function () { let o = new Object(); o.field = 5; Object.freeze(o); return o; })()")
+	want := 5
+	if got := o.Get("field").Int(); want != got {
+		t.Errorf("got %#v, want %#v", got, want)
+	}
+}
+
 func TestTypedArrayOf(t *testing.T) {
 	testTypedArrayOf(t, "[]int8", []int8{0, -42, 0}, -42)
 	testTypedArrayOf(t, "[]int16", []int16{0, -42, 0}, -42)