[wasm] Fix asm.js toString for modules with imports

Fix the string representation of functions exported from asm.js modules
in the presence of imported functions.

R=jkummerow@chromium.org

Bug: chromium:1045767, chromium:667678
Change-Id: I55714252036511598eeec7fe7b81985213f7e4f2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2022142Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65993}

src/objects/js-objects.cc

@@ -5549,7 +5549,7 @@ Handle<String> JSFunction::ToString(Handle<JSFunction> function) {
     if (is_asmjs_module(module)) {
       std::pair<int, int> offsets =
           module->asm_js_offset_information->GetFunctionOffsets(
-              function_data->function_index());
+              function_data->function_index() - module->num_imported_functions);
       Handle<String> source(
           String::cast(Script::cast(shared_info->script()).source()), isolate);
       return isolate->factory()->NewSubString(source, offsets.first,

test/mjsunit/wasm/asm-wasm-names.js

@@ -14,3 +14,15 @@ var func = Module({}, {}, new ArrayBuffer(65536)).bar;
 assertEquals("Module", Module.name);
 assertEquals("foo", func.name);
 assertEquals("function foo() {}", func.toString());
+
+function imp() {}
+function Module2(stdlib, imports) {
+  "use asm";
+  var imp = imports.imp;
+  function bar() {}
+  return {bar: bar};
+}
+
+var bar = Module2({}, {imp: imp}).bar;
+assertEquals("bar", bar.name);
+assertEquals("function bar() {}", bar.toString());