[parser] Ensure asm.js modules always allocate context.

The deserialization of the {Scope::asm_module} predicate relies on a
context being present for such modules. This ensures we always allocate
such a context, even in cases where no variables are allocated in it.

R=neis@chromium.org
TEST=cctest/test-parsing/AsmModuleFlag
BUG=v8:5653

Review-Url: https://codereview.chromium.org/2561103004
Cr-Commit-Position: refs/heads/master@{#41611}

src/ast/scopeinfo.cc

@@ -445,6 +445,7 @@ int ScopeInfo::ContextLength() {
                        (scope_type() == BLOCK_SCOPE && CallsSloppyEval() &&
                         is_declaration_scope()) ||
                        (scope_type() == FUNCTION_SCOPE && CallsSloppyEval()) ||
+                       (scope_type() == FUNCTION_SCOPE && IsAsmModule()) ||
                        scope_type() == MODULE_SCOPE;
 
     if (has_context) {

src/ast/scopes.cc

@@ -2061,9 +2061,9 @@ void Scope::AllocateVariablesRecursively() {
   // Force allocation of a context for this scope if necessary. For a 'with'
   // scope and for a function scope that makes an 'eval' call we need a context,
   // even if no local variables were statically allocated in the scope.
-  // Likewise for modules.
+  // Likewise for modules and function scopes representing asm.js modules.
   bool must_have_context =
-      is_with_scope() || is_module_scope() ||
+      is_with_scope() || is_module_scope() || IsAsmModule() ||
       (is_function_scope() && calls_sloppy_eval()) ||
       (is_block_scope() && is_declaration_scope() && calls_sloppy_eval());
 

test/cctest/test-parsing.cc

@@ -3446,6 +3446,7 @@ i::Scope* DeserializeFunctionScope(i::Isolate* isolate, i::Zone* zone,
 }  // namespace
 
 TEST(AsmModuleFlag) {
+  i::FLAG_validate_asm = false;
   i::Isolate* isolate = CcTest::i_isolate();
   i::HandleScope scope(isolate);
   LocalContext env;
@@ -3453,8 +3454,7 @@ TEST(AsmModuleFlag) {
   const char* src =
       "function m() {"
       "  'use asm';"
-      "  var x = 0;"
-      "  function f() { return x };"
+      "  function f() { return 0 };"
       "  return { f:f };"
       "}"
       "m();";