[wasm] Fix prototype property of exported functions

According to the spec, exported wasm functions should not have a
[[Construct]] method, hence they don't have a prototype.

R=bmeurer@chromium.org
CC=​titzer@chromium.org

Bug: v8:7503
Change-Id: I9e142d65a80c0ef6dbd743421771f194c2d50614
Reviewed-on: https://chromium-review.googlesource.com/939782Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51622}

src/wasm/wasm-objects.cc

@@ -853,8 +853,11 @@ Handle<WasmExportedFunction> WasmExportedFunction::New(
   shared->set_length(arity);
   shared->set_internal_formal_parameter_count(arity);
   NewFunctionArgs args = NewFunctionArgs::ForWasm(
-      name, export_wrapper, isolate->sloppy_function_map());
+      name, export_wrapper, isolate->sloppy_function_without_prototype_map());
   Handle<JSFunction> js_function = isolate->factory()->NewFunction(args);
+  // According to the spec, exported functions should not have a [[Construct]]
+  // method.
+  DCHECK(!js_function->IsConstructor());
 
   js_function->set_shared(*shared);
   Handle<Symbol> instance_symbol(isolate->factory()->wasm_instance_symbol());

test/mjsunit/wasm/function-prototype.js

@@ -17,8 +17,9 @@ load("test/mjsunit/wasm/wasm-module-builder.js");
   var func = builder.instantiate().exports.nine;
 
   // Check type and existence of prototype
-  assertEquals("function", typeof func.apply);
-  assertTrue(func.prototype != undefined);
+  assertEquals('function', typeof func);
+  assertEquals('function', typeof func.apply);
+  assertEquals('prototype' in func, false);
   assertEquals(String(f.index), func.name);
   assertEquals(undefined, func.displayName);