webassembly/objjsproxy: Fix binding of self to JavaScript methods.

Fixes a bug in the binding of self/this to JavaScript methods.

The new semantics match Pyodide's behaviour, at least for the included
tests.

Signed-off-by: Damien George <damien@micropython.org>

tests/ports/webassembly/method_bind_behaviour.mjs

@@ -0,0 +1,43 @@
+// Test how JavaScript binds self/this when methods are called from Python.
+
+const mp = await (await import(process.argv[2])).loadMicroPython();
+
+// Test accessing and calling JavaScript methods from Python.
+mp.runPython(`
+import js
+
+# Get the push method to call later on.
+push = js.Array.prototype.push
+
+# Create initial array.
+ar = js.Array(1, 2)
+js.console.log(ar)
+
+# Add an element using a method (should implicitly supply "ar" as context).
+print(ar.push(3))
+js.console.log(ar)
+
+# Add an element using prototype function, need to explicitly provide "ar" as context.
+print(push.call(ar, 4))
+js.console.log(ar)
+
+# Add an element using a method with call and explicit context.
+print(ar.push.call(ar, 5))
+js.console.log(ar)
+
+# Add an element using a different instances method with call and explicit context.
+print(js.Array().push.call(ar, 6))
+js.console.log(ar)
+`);
+
+// Test assigning Python functions to JavaScript objects, and using them like a method.
+mp.runPython(`
+import js
+
+a = js.Object()
+a.meth1 = lambda *x: print("meth1", x)
+a.meth1(1, 2)
+
+js.Object.prototype.meth2 = lambda *x: print("meth2", x)
+a.meth2(3, 4)
+`);